RPM source_date_epoch test should check rpmbuild version, not system rpm

We have a skip in this test, but we dumbly check the system "rpm" binary than the rpmbuild binary used to build the RPM in the first place. We have skipping logic here:

https://github.com/bazelbuild/rules_pkg/blob/9748dfa1a9a884d4107e5980f4bb3fa330f18b15/pkg/tests/rpm/source_date_epoch/rpm_contents_vs_manifest_test.py#L57-L64

But it could not be working for numerous reasons. Investigate further.

_Originally posted by @nacl in https://github.com/bazelbuild/rules_pkg/pull/443#discussion_r742274241_