[llvm] On Windows, make the release script work with the local git checkout (PR #78273)

[via llvm-commits]

zmodem wrote:

> I am fine with the changes here - but I wonder about the flag name `--test`. I didn't understand what it did until I really read the help text, is there a better name, maybe `--local` or `--no-download` or something.

+1, how about `--skip-checkout`?

https://github.com/llvm/llvm-project/pull/78273