[v12.x] build: pin macOS GitHub runner to macos-10.15#41124

richardlau merged 1 commit intonodejs:v12.x-stagingnodejs/node:v12.x-stagingfrom

richardlau:v12.x-staging-macos-runnerrichardlau/node-1:v12.x-staging-macos-runnerCopy head branch name to clipboard