Opt-out of PATH changes

[pcj]

Bazelisk changes the PATH variable, which has deleterious consequences for nix-based builds, and it seems like a number of other scenarios. There should be a way to ensure that all environment variables are passed through faithfully without modification to the underlying bazel executable.

Related:

• https://github.com/bazelbuild/bazelisk/issues/471
• https://github.com/bazelbuild/bazelisk/issues/301

[keith]

2 practical workarounds for this today:

1. use tools/bazel and re-exec $BAZEL_REAL with env PATH=whatever
2. --repo_env=PATH=... --action_env=PATH=... --test_env=PATH=...