[Request] Add ability to trigger test builds

[xiota]

From @dr460nf1r3:

how about implementing build tests via MR? Like:

• passing the builder another branch to source pkgbuilds from
• test building the package but don't actually deploy them
• maybe transfer the archive from the landing zone to the staging repo location which @stackofprimes mentioned
• make passing builds a requirement for merging MRs
  I imagine this decreases maintainers time spent with test building packages and also prevents bad PKGBUILDs ending up in main

This could also run a few other checks to ensure noticing potential issues.

Another benefit could be being the base for staging rebuilds, eg. we bump packages and move them to the prod repo once the MR has been merged?

Another implementation possibility is to have a command line utility that is run after logging on to the build server, similar to chaotic v3.

[xiota]

Also, maybe, ability to test with and without interfere.