Auto merge of #22175 - servo:jdm-patch-19, r=jdm

Fix linux-rel-intermittent build.

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etc/ci/buildbot_steps.yml

@@ -74,6 +74,10 @@ mac-nightly:
   - ./etc/ci/update-wpt-checkout cleanup
 
 linux-rel-intermittent:
+ env:
+   CC: gcc-5
+   CXX: g++-5
+ commands:
   - ./mach clean-nightlies --keep 3 --force
   - ./mach clean-cargo-cache --keep 3 --force
   - ./etc/ci/clean_build_artifacts.sh