Auto merge of #19884 - aneeshusa:fix-mutation-test, r=jdm

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.

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- [x] `./mach test-tidy` does not report any errors
- [x] These changes should fix #19821 (github issue number if applicable).

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