Disable pull-request github workflows

[tobiasdiez]

Currently, there are a few github workflows triggered by pull-requests (e.g. ci-linux and ci-macos). As they take too much time to be invoked on every pull request, this triggers should be removed.

[mkoeppe]

Taking care of this in https://trac.sagemath.org/ticket/34942

[tobiasdiez]

What is the plan to get this ticket now merged? If one allows PRs to the new repo (without merging this ticket first), then the ci will be overloaded quite quickly. Can you open a PR with your commits and merge it before opening up the new repo for the "public"?

[mkoeppe]

@vbraun Would you be able to prepare a 9.8.rc1 that includes https://trac.sagemath.org/ticket/34942?