Do not run CI on beta or nightly (#352) #14

Job	Run time
windows-build-and-test-stable / build-and-test	3m 33s
windows-build-and-test-msrv / build-and-test	3m 47s
windows-coverage-stable / coverage	1m 47s
windows-lint-stable / lint	2m 31s
	11m 38s

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