Streamline our CI and testing setup

[YJDoc2]

After merge of #2536 , we now have build and testing for x86,aarch64 , libc and gnu with various combinations. These roughly launch 27 CI runs, as can be seen in the commit of #2584 , which are a lot of checks unless we are ready to merge.
For example, in #2585 , which change docs, but still runs all PRs.

Even with adding conditional checks to run the build/test CI only on code changes, 27 CI are a lot to run on each commit in PR unless we are ready to merge (i.e. code is reviewed and changes are done etc.) . What I have in mind is to have a label based trigger, so when a specific label (eg. run-tests) is added to PR, only then the checks are run. (We'll also have to consider dependabot PRs for automatic merges). We can keep some basic checks to run on each commit such as linting and formatting, maybe a single build for x86+gnu or such, so that we know the commit is at least compiling correctly.

More suggestions are welcome!

cc : @containers/youki-maintainers 👀