servo: Merge #19326 - Use same build environment during mutation testing (from servo:jdm-patch-2); r=asajeffrey

ccache complains that the C++ compiler doesn't match the cached version.

Source-Repo: https://github.com/servo/servo
Source-Revision: 93c2a5da89b7413837e61ec92ed529debd4cf5f8

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 4c88ae2dabf7ec0298843d19e910aef69952723d

servo/etc/ci/buildbot_steps.yml

@@ -68,7 +68,7 @@ linux-rel-nogate:
   - ./mach clean-nightlies --keep 3 --force
   - env CC=gcc-5 CXX=g++-5 ./mach build --release
   - python ./etc/ci/chaos_monkey_test.py
-  - bash ./etc/ci/mutation_test.sh
+  - env CC=gcc-5 CXX=g++-5 bash ./etc/ci/mutation_test.sh
 
 mac-rel-intermittent:
   - ./mach clean-nightlies --keep 3 --force