fix for DefaultProceessController

[gopito]

When I use DefaultProcessController with applesimutils
var processController = DefaultProcessController( subprocess: Subprocess(arguments: [ "/usr/local/bin/applesimutils", "-id", "EB6808EB-9181-45EF-A31F-668EE42456C0", "-b", "my.bundle.id", "-sp", "notifications=YES, photos=YES, camera=YES, userTracking=NO, microphone=YES, location=always" ]) )

I get exit code 255. I find out that it is because of environment variables that we pass to process. Even default empty array [:] leads to error exit code.

[beefon]

We've discussed this internally. It is not clear why this can solve any possible problem with that applesim. I think it worth debugging it instead of making a change into CLT.
Besides, setting subprocess.environment to .current should do exactly the same thing you did in this PR.
Also, this change disallows passing an empty env into subprocess.