bpo-45629: Improve test.support.skip_if_buildbot (GH-31094)

bpo-45629: Improve test.support.skip_if_buildbot (GH-31094)

It was added as part of #29222 to avoid running freeze tool tests on the
buildbots but the logic was wrong so it did not skip tests on typical posix
setup buildbots where the worker is launched from cron via an @reboot task and
thus have no USER environment variable.  This uses the canonical
`getpass.getuser()` API rather than rolling its own attempt.