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.

etc/ci/buildbot_steps.yml

@@ -96,6 +96,10 @@ mac-rel-intermittent:
   - ./etc/ci/check_intermittents.sh --log-raw intermittents.log
 
 linux-dev:
+  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
@@ -112,6 +116,10 @@ linux-dev:
     - bash ./etc/ci/check_no_panic.sh
 
 linux-rel-wpt:
+  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
@@ -121,6 +129,10 @@ linux-rel-wpt:
     - ./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:
+  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