[llvm] [CI] Skip libcxx in non-llvm repo (PR #73282)

[Tom Stellard via llvm-commits]

tstellar wrote:

@EricWF Do you test these in the libcxx org?  If yes, then you might want to do a follow on commit that enables it for this org.

@jsji I think this is fine to commit now given the large impact it has on forks.

https://github.com/llvm/llvm-project/pull/73282