Only run Autobuild if necessary

[ann0see]

Has this feature been discussed and generally agreed?
Related to #1806
No. However @ignotus666 developed something comparable on the website: jamulussoftware/jamuluswebsite#586

Describe the solution you'd like
Currently the CI runs even if it is not needed (e.g. twice if there's a merge, if .md files which do not contribute to the build are changed,...).
The CI should only run if necessary.

This means if we change...

• .md files
• merge to master and we already had a PR run
• .gitignore/.clang-format* is changed, we don't need to run ci at all

... the CI doesn't have to run

Every PR does not need to:

• Start a legacy build (since there's a beta release before each build, we'd see if something breaks on older Qt versions)
• JACK build (not yet implemented)

Note:
if: ${{ contains(steps.jamulus-build-vars.outputs.PUBLISH_TO_RELEASE, 'true') }}

specifies if we have a release or just a normal build.
... but every (beta, nightly,...) release needs to build them all.

Why I consider this as important

• We're wasting resources (storage, power) --> environment
• We're wasting time (there should be no need to run multiple builds which take ages all over the exactly same code)

[pljones]

I still like my .buildignore idea.

[ann0see]

Yes maybe. You can have a look at the work of @ignotus666 on the website.

[ann0see]

Closing in terms of #2282