Pass default_shell_env through actions

[keith]

Through the investigation of
bazelbuild/bazel#12049 one of the things I
discovered was that when using actions.run there are 2 options for
environment variables. use_default_shell_env = True is recommended,
but cannot be used if you want to also pass env to the actions. To
support Xcode version selection we have to pass env with those
variables. Without the default shell env, we only get the environment
variables defined in the crosstool, but not those passed with
--action_env. This now adds variables passed as
--action_env=FOO=BAR, but not those passed as --action_env=FOO
(where the value is supposed to pass through).

This is useful to ensure a few things:

1. The default PATH things are executed with includes /usr/local/bin.
   This can result in pollution of binaries from homebrew. Previously
   there was no way to limit this
2. This should be a good replacement for using custom Swift toolchains.
   Currently those environment variables only apply to some actions
   (excluding those from bazel) using --action_env=TOOLCHAINS=foo
   should work better than the current solution (this change can be made
   as a followup)

[segiddins]

So I was thinking of an alternative -- instead of trying to merge in the configuration's default shell env, what if we used env to add in the env vars we want, and call the executables that way?

[keith]

Use which env to add them where?

[segiddins]

/usr/bin/env FOO=BAR ... -- executable args...

[keith]

ah, I think piping that through would probably be a bit more difficult than this. I'm not sure we want to compete with bazel's logic for this

[brentleyjones]

FYI: I've been using this with --action_env=TOOLCHAINS=$TOOLCHAINS to great effect.

[keith]

#1114