CI: don't run macOS 10.13 automatically
Trying to set things up so macOS 10.13.6 only runs when we click the little button. Its distracting when it fails, which is does a lot b/c of #1664 (closed) and #2023 (closed).
Trying to set things up so macOS 10.13.6 only runs when we click the little button. Its distracting when it fails, which is does a lot b/c of #1664 (closed) and #2023 (closed).