Automatically clean old nightlies

Add a new `-keep` flag to `./mach clean-nightlies` to control how many
nightlies to keep, which keeps the n most recent nightlies found in git.

Use this to clean old nightlies at the start of each build,
currently keeping 3 nightlies at a time.

etc/ci/buildbot_steps.yml

@@ -1,4 +1,5 @@
 mac-rel-wpt1:
+  - ./mach clean-nightlies --keep 3 --force
   - ./mach build --release
   - ./mach test-wpt-failure
   - ./mach test-wpt --release --processes 8 --total-chunks 2 --this-chunk 1 --log-raw test-wpt.log --log-errorsummary wpt-errorsummary.log --always-succeed
@@ -9,12 +10,14 @@ mac-rel-wpt1:
   - bash ./etc/ci/manifest_changed.sh
 
 mac-rel-wpt2:
+  - ./mach clean-nightlies --keep 3 --force
   - ./mach build --release
   - ./mach test-wpt --release --processes 8 --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 --use-tracker
   - ./mach build-geckolib --release
 
 mac-dev-unit:
+  - ./mach clean-nightlies --keep 3 --force
   - env SERVO_RUSTC_LLVM_ASSERTIONS=1 ./mach build --dev
   - env SERVO_RUSTC_LLVM_ASSERTIONS=1 ./mach test-unit
   - env SERVO_RUSTC_LLVM_ASSERTIONS=1 ./mach package --dev
@@ -24,6 +27,7 @@ mac-dev-unit:
   - bash ./etc/ci/manifest_changed.sh
 
 mac-rel-css:
+  - ./mach clean-nightlies --keep 3 --force
   - ./mach build --release
   - ./mach test-css --release --processes 4 --log-raw test-css.log --log-errorsummary css-errorsummary.log --always-succeed
   - ./mach filter-intermittents css-errorsummary.log --log-intermittents intermittents.log --log-filteredsummary filtered-css-errorsummary.log --use-tracker
@@ -31,20 +35,24 @@ mac-rel-css:
   - bash ./etc/ci/manifest_changed.sh
 
 mac-nightly:
+  - ./mach clean-nightlies --keep 3 --force
   - ./mach build --release
   - ./mach package --release
   - ./etc/ci/upload_nightly.sh mac
   - ./etc/ci/upload_nightly.sh macbrew
 
 linux-rel-intermittent:
+  - ./mach clean-nightlies --keep 3 --force
   - ./mach build --release
   - ./etc/ci/check_intermittents.sh --log-raw intermittents.log
 
 mac-rel-intermittent:
+  - ./mach clean-nightlies --keep 3 --force
   - ./mach build --release
   - ./etc/ci/check_intermittents.sh --log-raw intermittents.log
 
 linux-dev:
+  - ./mach clean-nightlies --keep 3 --force
   - ./mach test-tidy --no-progress --all
   - ./mach test-tidy --no-progress --self-test
   - env SERVO_RUSTC_LLVM_ASSERTIONS=1 ./mach build --dev
@@ -59,6 +67,7 @@ linux-dev:
   - bash ./etc/ci/check_no_panic.sh
 
 linux-rel-wpt:
+  - ./mach clean-nightlies --keep 3 --force
   - ./mach build --release --with-debug-assertions
   - ./mach test-wpt-failure
   - ./mach test-wpt --release --processes 24 --log-raw test-wpt.log --log-errorsummary wpt-errorsummary.log --always-succeed
@@ -66,6 +75,7 @@ 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:
+  - ./mach clean-nightlies --keep 3 --force
   - ./mach build --release --with-debug-assertions
   - ./mach test-css --release --processes 16 --log-raw test-css.log --log-errorsummary css-errorsummary.log --always-succeed
   - ./mach filter-intermittents css-errorsummary.log --log-intermittents intermittents.log --log-filteredsummary filtered-css-errorsummary.log --use-tracker
@@ -76,11 +86,13 @@ linux-rel-css:
   - bash ./etc/ci/manifest_changed.sh
 
 linux-nightly:
+  - ./mach clean-nightlies --keep 3 --force
   - ./mach build --release
   - ./mach package --release
   - ./etc/ci/upload_nightly.sh linux
 
 android:
+  - ./mach clean-nightlies --keep 3 --force
   - env SERVO_RUSTC_LLVM_ASSERTIONS=1 ./mach build --android --dev
   - env SERVO_RUSTC_LLVM_ASSERTIONS=1 ./mach package --android --dev
   - bash ./etc/ci/lockfile_changed.sh
@@ -88,27 +100,32 @@ android:
   - python ./etc/ci/check_dynamic_symbols.py
 
 android-nightly:
+  - ./mach clean-nightlies --keep 3 --force
   - env SERVO_RUSTC_LLVM_ASSERTIONS=1 ./mach build --android --release
   - env SERVO_RUSTC_LLVM_ASSERTIONS=1 ./mach package --android --release
   - ./etc/ci/upload_nightly.sh android
 
 arm32:
+  - ./mach clean-nightlies --keep 3 --force
   - env SERVO_RUSTC_LLVM_ASSERTIONS=1 ./mach build --rel --target=arm-unknown-linux-gnueabihf
   - bash ./etc/ci/lockfile_changed.sh
   - bash ./etc/ci/manifest_changed.sh
 
 arm64:
+  - ./mach clean-nightlies --keep 3 --force
   - env SERVO_RUSTC_LLVM_ASSERTIONS=1 ./mach build --rel --target=aarch64-unknown-linux-gnu
   - bash ./etc/ci/lockfile_changed.sh
   - bash ./etc/ci/manifest_changed.sh
 
 windows-msvc-dev:
+  - mach.bat clean-nightlies --keep 3 --force
   - mach.bat build --dev
   - mach.bat test-unit
   - mach.bat package --dev
   - mach.bat build-geckolib
 
 windows-msvc-nightly:
+  - mach.bat clean-nightlies --keep 3 --force
   - mach.bat build --release
   - mach.bat package --release
   - bash -l ./etc/ci/upload_nightly.sh windows-msvc