Support for toolchain env integration tests

I noticed that when using `analysistest` the `env` you get from actions does not include any environment variables added by a cc toolchain. this makes it particularly difficult to test when env is added / missing based on feature differences. Right now it's easy to test argv for that case but not env.

I'm curious if google is doing anything internally for this case to make testing this easier? but maybe you just don't use env much for this case?