ci: only run linux in PR by default, and run the rest on labels

[ruben-arts]

This PR will do the following:

• Disable the build and test on all platforms except linux x86_64
• Allow the developer to enable the other platforms through labels like ci:all ci:win ci:macos.

The fast runners became to expensive to run on every commit and especially windows. Thus this should allow for quick PR's, to save guard ourselfs we leave the auto merge requirements to include all platforms to be tested, otherwise the maintainer can force merge with the risk of breaking main for non linux platforms. So it will then be their problem to fix it.

[hoxbro]

Just a drive-by comment, but for the required jobs which is no longer run by default, you could add a collector at the end and make that the required one

  collector:
    name: Collect Result
    needs: [build, cargo-test]
    if: always()
    runs-on: ubuntu-latest
    steps:
      - name: check for failures
        if: contains(needs.*.result, 'failure') || contains(needs.*.result, 'cancelled')
        run: echo job failed && exit 1

[ruben-arts]

Just a drive-by comment, but for the required jobs which is no longer run by default, you could add a collector at the end and make that the required one

Oh thanks @hoxbro, thats a good idea. The problem we have here is that you can't run based on the "if" through a matrix. I'm planning to rewrite this into different jobs. Or do you have a good idea how we could build this?

[hoxbro]

I haven't looked into the details, but you could dynamically create the matrix in another step and unpack it when needed.

I have done something like that here: https://github.com/holoviz/holoviews/blob/519f720ed5e9affbfdb9ac3efed39f7a00e7fef4/.github/workflows/test.yaml#L121

[ruben-arts]

@hoxbro The setup job seems like a smart move!