specify runner os versions #887

Job	Run time
cancel-old-build	3s
build-win32 (debug, static_lib)	58s
build-darwin (debug, static_lib, x64)	14s
build-win32 (debug, shared_lib)	50s
build-linux (debug, static_lib)	25s
build-darwin (debug, static_lib, arm64)	1s
build-win32 (debug, cmd)	50s
build-linux (debug, shared_lib)	27s
test-darwin (debug, arm64)	29s
build-darwin (debug, shared_lib, x64)	15s
build-win32 (release, static_lib)	1m 44s
build-linux (debug, cmd)	34s
test-darwin (debug, x64)	17s
build-darwin (debug, shared_lib, arm64)	19s
build-win32 (release, shared_lib)	1m 36s
build-linux (release, static_lib)	1m 20s
test-darwin (release, arm64)	24s
build-darwin (debug, cmd, x64)	17s
build-win32 (release, cmd)	2m 14s
build-linux (release, shared_lib)	46s
test-darwin (release, x64)	28s
build-darwin (debug, cmd, arm64)	27s
build-linux (release, cmd)	50s
build-darwin (release, static_lib, x64)	1s
test-linux (debug)	42s
test-linux (release)	51s
test-win32 (debug)	50s
build-darwin (release, static_lib, arm64)	1s
test-win32 (release)	1m 46s
build-darwin (release, shared_lib, x64)	1s
build-darwin (release, shared_lib, arm64)	1s
build-darwin (release, cmd, x64)	1s
build-darwin (release, cmd, arm64)	1s
dist-darwin	0s
dist-linux	7s
dist-win32	15s
	20m 25s

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