Merge branch 'cb/ci-make-p4-optional'

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