Pin MacOS version in GH actions #12326

Job	Run time
Test suite with py38-ubuntu, mypyc-compiled	19m 32s
Test mypyc suite with 32-bit Python	8m 11s
Test suite with py38-windows-64	26m 16s
Test suite with py39-ubuntu	20m 44s
Test suite with py310-ubuntu	17m 19s
Test suite with py311-ubuntu, mypyc-compiled	17m 44s
Test suite with py312-ubuntu, mypyc-compiled	20m 21s
mypyc runtime tests with py39-macos	5m 29s
mypyc runtime tests with py38-debug-build-ubuntu	5m 14s
Type check our own code (py38-ubuntu)	1m 50s
Type check our own code (py38-windows-64)	2m 32s
Formatting and code style with Black + ruff	38s
	2h 25m 50s

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