feat: dispatch event to `flarum/installation-packages` on release

[SychO9]

Changes proposed in this pull request: Auto trigger build of installation packages on flarum/installation-packages.

• Needs a PACKAGES_BUILD_TOKEN secret added first. The token must have permission to trigger the workfow_dispatch event on the target repository.
• Then needs to be tested.

Necessity

• [ ] Has the problem that is being solved here been clearly explained?
• [ ] If applicable, have various options for solving this problem been considered?
• [ ] For core PRs, does this need to be in core, or could it be in an extension?
• [ ] Are we willing to maintain this for years / potentially forever?

Confirmed

• [ ] Frontend changes: tested on a local Flarum installation.
• [ ] Backend changes: tests are green (run composer test).
• [ ] Core developer confirmed locally this works as intended.
• [ ] Tests have been added, or are not appropriate here.

[luceos]

Is the idea behind your workflow to run after the release, cq the version has been tagged? Because the prepare release workflow only gathers changes, commits, contributors and donators. It doesn't actually tag (yet) because tags need changelogs..

[SychO9]

Argh.. true, you run the prepare release workflow first, then tag.

That's fine we can always trigger the tarball build manually as well.

[SychO9]

Actually, let me just switch to a separate workflow that runs on release