Allow for cross-platform remote execution (i.e. from OSX to Linux)

[stuhood]

This was designed in https://docs.google.com/document/d/1hS0n6vp-hBxy4Xo-q9QA_elofzQtOubjfN4nSeIgp90/edit#

[stuhood]

I've added a section to https://docs.google.com/document/d/1vXRHWK7ZjAIp2BxWLwRYm1QOKDeXx02ONQWvXDloxkg/edit#heading=h.w0bkj2kpqzfq which describes how environment matchers should likely support this.

[stuhood]

I've confirmed that this is working via #7735: huzzah! Thanks @Eric-Arellano, @chrisjrn, @tdyas.

Before calling this resolved though, I'll need to think about how we can migrate from the current "--remote-execution enables remote execution everywhere" world, to a world where that is accomplished via environments, as described in #17096 (which is not currently accurate).

[stuhood]

Ok, after talking with @Eric-Arellano, it sounds like they had already included some backwards compatibility for the --remote-execution flag. So this one was probably already resolvable as soon as they made that change =)

My next change is going to be #17129 to help clarify which scopes environments need to be lifted from.