process: replace getuid with rustix::process::getuid

[Franklin-Qi]

replace nix::unistd::getuid with rustix::process::getuid

closed: #12042

[oech3]

Is uid_t correct?

[oech3]

It would be nice to replace other get*id()'s (if it is possible with small diff).

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/rm/isatty (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/tail/retry is no longer failing!
Note: The gnu test tests/expand/bounded-memory is now being skipped but was previously passing.
Note: The gnu test tests/env/env-signal-handler was skipped on 'main' but is now failing.

[xtqqczze]

Is uid_t correct?

It should be rustix::process::RawUid