servo: Merge #19740 - Allow warnings during mutation tests (from servo:jdm-patch-2); r=aneeshusa

Sometimes mutations include removing the only use of a variable. When we deny warnings globally, we inhibit these mutations that trigger new warnings.

Source-Repo: https://github.com/servo/servo
Source-Revision: 2b9dbc341502f5e9e1fb6e071c3d5119cc263a46

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : ea29866756cb97c1ffe903334e9d73a9a30dbd88

servo/etc/ci/buildbot_steps.yml

@@ -73,7 +73,7 @@ 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
-  - env CC=gcc-5 CXX=g++-5 bash ./etc/ci/mutation_test.sh
+  - env CC=gcc-5 CXX=g++-5 RUSTFLAGS= bash ./etc/ci/mutation_test.sh
 
 mac-rel-intermittent:
   - ./mach clean-nightlies --keep 3 --force