pin macos test runners to macos-12 (#1195) (#1196) #16790

Job	Run time
test-metadata	3s
test-python	13m 31s
bigquery / python 3.8 / ubuntu-latest	42m 8s
bigquery / python 3.9 / ubuntu-latest	1h 4m 3s
bigquery / python 3.10 / ubuntu-latest	50m 14s
bigquery / python 3.11 / ubuntu-latest	48m 11s
bigquery / python 3.8 / windows-latest	51m 33s
bigquery / python 3.8 / macos-12	50m 20s
require-label-comment	7s
	5h 20m 10s

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