Auto merge of #19883 - aneeshusa:reenable-sccache-partially, r=jdm

Re-enable sccache for Linux builds

As far as I know, sccache is working properly
on the non-cross-compiling Linux builders.
For safety, only enable it for the builders that run on PRs,
to avoid breaking our nightly generation and scheduled test runs.

This will also allow testing new versions of sccache more easily.
This implements my suggestion from https://github.com/servo/servo/pull/19858#issuecomment-360514782, and should also let us
handle testing a new sccache: https://github.com/rust-lang/rust/issues/42867#issuecomment-358692077 (our current version of sccache [seems to be 2018-01-09](f50214b8fa/servo-build-dependencies/ci-map.jinja (L5))).

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- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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etc/ci/buildbot_steps.yml

@@ -96,40 +96,52 @@ mac-rel-intermittent:
   - ./etc/ci/check_intermittents.sh --log-raw intermittents.log
 
 linux-dev:
-  - ./mach clean-nightlies --keep 3 --force
-  - ./mach clean-cargo-cache --keep 3 --force
-  - ./mach test-tidy --no-progress --all
-  - ./mach test-tidy --no-progress --self-test
-  - ./mach build --dev
-  - ./mach test-unit
-  - ./mach package --dev
-  - ./mach build-cef
-  - ./mach build --dev --no-default-features --features default-except-unstable
-  - ./mach build-geckolib
-  - ./mach test-stylo
-  - bash ./etc/ci/lockfile_changed.sh
-  - bash ./etc/ci/manifest_changed.sh
-  - bash ./etc/ci/check_no_panic.sh
+  env:
+    CCACHE: sccache
+    RUSTC_WRAPPER: sccache
+  commands:
+    - ./mach clean-nightlies --keep 3 --force
+    - ./mach clean-cargo-cache --keep 3 --force
+    - ./mach test-tidy --no-progress --all
+    - ./mach test-tidy --no-progress --self-test
+    - ./mach build --dev
+    - ./mach test-unit
+    - ./mach package --dev
+    - ./mach build-cef
+    - ./mach build --dev --no-default-features --features default-except-unstable
+    - ./mach build-geckolib
+    - ./mach test-stylo
+    - bash ./etc/ci/lockfile_changed.sh
+    - bash ./etc/ci/manifest_changed.sh
+    - bash ./etc/ci/check_no_panic.sh
 
 linux-rel-wpt:
-  - ./mach clean-nightlies --keep 3 --force
-  - ./mach clean-cargo-cache --keep 3 --force
-  - ./mach build --release --with-debug-assertions
-  - ./mach test-wpt-failure
-  - ./mach test-wpt --release --processes 24 --total-chunks 2 --this-chunk 1 --log-raw test-wpt.log --log-errorsummary wpt-errorsummary.log --always-succeed
-  - ./mach filter-intermittents wpt-errorsummary.log --log-intermittents intermittents.log --log-filteredsummary filtered-wpt-errorsummary.log --tracker-api default --reporter-api default
-  - ./mach test-wpt --release --binary-arg=--multiprocess --processes 24 --log-raw test-wpt-mp.log --log-errorsummary wpt-mp-errorsummary.log eventsource
+  env:
+    CCACHE: sccache
+    RUSTC_WRAPPER: sccache
+  commands:
+    - ./mach clean-nightlies --keep 3 --force
+    - ./mach clean-cargo-cache --keep 3 --force
+    - ./mach build --release --with-debug-assertions
+    - ./mach test-wpt-failure
+    - ./mach test-wpt --release --processes 24 --total-chunks 2 --this-chunk 1 --log-raw test-wpt.log --log-errorsummary wpt-errorsummary.log --always-succeed
+    - ./mach filter-intermittents wpt-errorsummary.log --log-intermittents intermittents.log --log-filteredsummary filtered-wpt-errorsummary.log --tracker-api default --reporter-api default
+    - ./mach test-wpt --release --binary-arg=--multiprocess --processes 24 --log-raw test-wpt-mp.log --log-errorsummary wpt-mp-errorsummary.log eventsource
 
 linux-rel-css:
-  - ./mach clean-nightlies --keep 3 --force
-  - ./mach clean-cargo-cache --keep 3 --force
-  - ./mach build --release --with-debug-assertions
-  - ./mach test-wpt --release --processes 24 --total-chunks 2 --this-chunk 2 --log-raw test-wpt.log --log-errorsummary wpt-errorsummary.log --always-succeed
-  - ./mach filter-intermittents wpt-errorsummary.log --log-intermittents intermittents.log --log-filteredsummary filtered-wpt-errorsummary.log --tracker-api default --reporter-api default
-  - ./mach build-geckolib --release
-  - ./mach test-stylo --release
-  - bash ./etc/ci/lockfile_changed.sh
-  - bash ./etc/ci/manifest_changed.sh
+  env:
+    CCACHE: sccache
+    RUSTC_WRAPPER: sccache
+  commands:
+    - ./mach clean-nightlies --keep 3 --force
+    - ./mach clean-cargo-cache --keep 3 --force
+    - ./mach build --release --with-debug-assertions
+    - ./mach test-wpt --release --processes 24 --total-chunks 2 --this-chunk 2 --log-raw test-wpt.log --log-errorsummary wpt-errorsummary.log --always-succeed
+    - ./mach filter-intermittents wpt-errorsummary.log --log-intermittents intermittents.log --log-filteredsummary filtered-wpt-errorsummary.log --tracker-api default --reporter-api default
+    - ./mach build-geckolib --release
+    - ./mach test-stylo --release
+    - bash ./etc/ci/lockfile_changed.sh
+    - bash ./etc/ci/manifest_changed.sh
 
 linux-nightly:
   - ./mach clean-nightlies --keep 3 --force