Run irctest in CI

[progval]

Based on https://github.com/inspircd/inspircd/blob/148a90535c1b061efcc13439056f8ab786086084/.github/workflows/ci-irctest.yml

It caught these regressions in the past:

• null dereference in configure_authd #396
• format_client_tags: Add missing null terminator #462
• Revert "msgbuf: duplicate the string during partial parse and constify line" #463
• m_part calls h_privmsg_channel hook without setting msgbuf #484

[progval]

I chose to make one of the existing builds (the fastest one) export its artifacts at the end for the irctest job. This minimizes CPU usage, but unnecessarily increases the wall-clock run time because the dependency means the test job waits for every build job to be done.

I can make the test jobs have its their own build concurrently to the other ones if you prefer.