experimental_strict_action_env doesn't do the right thing with remote builds

`BazelRuleClassProvider.java` contains the following TODO:

// TODO(ulfjack): The default PATH should be set from the exec platform, which may be different
// from the local machine. For now, this can be overridden with --action_env=PATH=, so
// at least there's a workaround.

This issue causes Bazel to set `PATH` to sane default for, say, Windows, even when making use of a Linux-based remote build cluster. Users must currently pass `--action_env=PATH=/bin:/usr/bin` in addition to `--experimental_strict_action_env` to make Bazel functional when doing builds on Linux from a Windows host.

In https://github.com/bazelbuild/bazel/pull/6263, it was suggested that a separate bug for this is filed.