`remote-run` should look in the environment for input/output paths as
well as considering command line arguments.
With this change, `test/Runtime/Paths.cpp` should work for remote testing
and device testing.
rdar://106294557
Not only does this give us a huge speed-up, it also works around a problem
where sftp doesn't always update the modification time, which causes random
test failures.
rdar://88179140
This is mostly important to allow remote-run to be used to upload
files generally, while still using the same interface and arguments
that will be used to actually run executables.
(instead of locally)
The primary use case I'm looking at is easier testing across macOS
versions, but it could also be used for cross-compilation tests in
general (someday).
