Add %posttrans scriptlet to RPM package

[k0walik]

RPM package specification defines %posttrans scriptlet, which can be particularly useful when upgrading a package. We should add it to the pkg_rpm build rule.

Reference: https://docs.fedoraproject.org/en-US/packaging-guidelines/Scriptlets/#ordering

[cgrindel]

@k0walik #797 just merged disabling the doc builds. I believe that those are causing all/most of the failures in your CI. Could you update your branch with the latest?

[cgrindel]

Overall, LGTM. A couple of comments:

• I did not notice a test leveraging postrans_scriptlet_file. Could you add one?
• Could you update your branch with the latest to address the doc_build failures?

[k0walik]

• I did not notice a test leveraging postrans_scriptlet_file. Could you add one?

Sure! I modified test/rpm/BUILD to test both xxx_scriptlet and xxx_scriptlet_file attributes - it looks like file variant was not tested at all.

• Could you update your branch with the latest to address the doc_build failures?

Done

[cgrindel]

LGTM. I won't merge it yet, so that @aiuto can take a look.

[aiuto]

This looks like it should work.
Thanks for improving the testing