Fix mutation test in CI by passing env vars correctly

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.

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