expand builder-dispatch workflow for release building

[Habbie]

Short description

This makes it easier to generate packages from any branch or tag for one or more distribution targets at a time. It also adds a flag for building releases.

To try this, go to Actions on a repo that has a copy of the yaml file on any branch. Then pick the target ("Trigger specific package build") on the left, open the "Run workflow" dropdown, pick your branch with the yaml file in "Use workflow from"; then check the rest of the fields and hit "Run workflow".

Once merged, this will simplify - it will no longer be necessary to first open the "Run workflow from" dropdown.

Note that this does not magically know what distributions are available for a given product or tag/branch.

Checklist

I have:

• [ ] read the CONTRIBUTING.md document
• [ ] compiled this code
• [ ] tested this code
• [ ] included documentation (including possible behaviour changes)
• [ ] documented the code
• [ ] added or modified regression test(s)
• [ ] added or modified unit test(s)
• [ ] checked that this code was merged to master

[Habbie]

Thanks! I'll squash this, move some of my text above to yaml comments, and then I'll merge.

[Habbie]

This accidentally duplicates some of the work in #10989. Will look into combining things.

[Habbie]

This accidentally duplicates some of the work in #10989. Will look into combining things.

Merging this now. 10989 has some very decent extra stuff, but is not currently mergeable in full.