servo: Merge #19884 - Fix mutation test in CI by passing env vars correctly (from aneeshusa:fix-mutation-test); r=jdm

Originally, buildbot would try to directly execute `RUSTFLAGS=` as a binary
since it was the first token in the command string, which won't work.

Fix this by using the `env` command to set the environment variables,
as there is currently no support for setting per-step environment variables
in the `buildbot_steps.yml` file.

<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [ ] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes should fix #19821 (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [x] These changes do not require tests because they change the CI configuration

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Source-Repo: https://github.com/servo/servo
Source-Revision: 025e5d773313e16ffe73ae63efc068b79b82d531

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : f1acdfbff9c0c0bf16dcf5ae029222b525e5169f

servo/etc/ci/buildbot_steps.yml

@@ -87,7 +87,7 @@ linux-rel-nogate:
   - ./mach clean-cargo-cache --keep 3 --force
   - ./mach build --release
   - python ./etc/ci/chaos_monkey_test.py
-  - RUSTFLAGS= bash ./etc/ci/mutation_test.sh
+  - env RUSTFLAGS= bash ./etc/ci/mutation_test.sh
 
 mac-rel-intermittent:
   - ./mach clean-nightlies --keep 3 --force