[Github] Only run github actions on main monorepo (#69774)

There are currently a couple jobs that run on all forks of LLVM too (if
there is a PR opened, or in the case of the documentation builds, upon
pushing to main). This isn't desired behavior. This commit disables that
behavior, forcing the jobs to not run if they aren't running against
llvm/llvm-project or a PR against that repo.