Re-organise/schedule workflows to limit runners usage

[blapie]

As mentioned in recent issues/prs, in order to reduce the runtime of tests and prevent abuse of resources provided by github-hosted runners (but also upcoming self-hosted runners), we are planning to change the scheduling of tests:

• we will still build all variants on pre/post-commit
• but only run test-cross on post-commit or simply once week (qemu being the obvious bottleneck)
• we will also disable manual triggering of workflows

Just wanted to give everyone a heads up in case someone has any objections in us doing that.

Hopefully that also gives us more freedom in extending the test coverage to more OS, compilers, options, ...

[blapie]

PR #533 is implementing the first suggested changes.

Leaving the discussion open to welcome any new requests regarding scheduling and organisation of workflows.