macOS CI jobs have been occasionally flaky due to tentative version
skew between perforce and the homebrew packager. Instead of
failing the whole CI job, just let it skip the p4 tests when this
happens.
* cb/ci-make-p4-optional:
ci: use https, not http to download binaries from perforce.com
ci: reintroduce prevention from perforce being quarantined in macOS
ci: avoid brew for installing perforce
ci: make failure to find perforce more user friendly
69e3d1e550