ci(.github/workflows/*): switch to self-hosted runners (#8177)
With this PR, mathlib builds on all branches will use the self-hosted runners that have the "pr" tag. One self-hosted runner is reserved for bors staging branch builds and does not have that tag.
The `build_fork` workflow has been added for use by external forks (and other Lean projects which might want to copy mathlib's CI).
Co-authored-by: Gabriel Ebner <gebner@gebner.org>
