CI: use better job names on GitHub Actions

[fingolfin]

From discussion in PR #4378 with @wilfwilson:

Currently the names of the GitHub Action jobs are semi-automatically generated and maybe not very nice / readable. They are also prone to change when their configuration changes even slightly, which is annoying because then one also has to adjust the branch protection rules...

This can be resolved by assigning a fixed name to each task manually; the current "automatic names" were simply the quickest way to get going, but they are a bit unwieldy anyway. So, we can just define a name or NAME or JOB_NAME or whatever for each job, which either gets a full custom string, or a partially custom one. I.e., we could then replace

name: ${{ matrix.test-suites }} - ${{ matrix.extra }} - ${{ matrix.os }}

by something like

name: ${{ matrix.os }} - ${{ matrix.name }}