[Backport 1.1.latest] Pin `macos` runners to `macos-12` (#787) #5504

Job	Run time
build packages	20s
flake8	1m 43s
unit test / python 3.8	1m 3s
verify packages / python 3.8 / ubuntu-latest	41s
verify packages / python 3.9 / ubuntu-latest	41s
verify packages / python 3.8 / macos-12	1m 0s
verify packages / python 3.9 / macos-12	55s
verify packages / python 3.8 / windows-latest	2m 12s
verify packages / python 3.9 / windows-latest	2m 14s
	10m 49s

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