Don't modify the environment when executing a CI command

[fmeum]

os.environ does not contain certain special environment variables on Windows (e.g. =C:) that are present when executing from cmd.exe.

[meteorcloudy]

We probably cannot remove this, because we do set a bunch of env vars to pass information to the Bazel binary:

https://github.com/search?q=repo%3Abazelbuild%2Fcontinuous-integration+os.environ+path%3A%2F%5Ebuildkite%5C%2Fbazelci.py%2F&type=code

[fmeum]

But isn't the default to inherit the current process environment, which should be equivalent to passing os.environ except for filtering out these weird variables?

[meteorcloudy]

Ah, that's a good point. I'll test this in our testing buildkite org.

Another concern is that this might mess up bisecting on windows, since the --client_env will shadow other failures.