get_user_id: use EUID instead of getpass.getuser() to resolve users

[b4ldr]

fixes #1577

[Byron]

No argument is made for why this should be preferred over the current implementation.

However, while checking what git does I noticed that it does also use getuid().

Thus there is nothing to fix, and changing it would be a breaking change as well which is another reason not to pursue this further. Thanks for your understanding.