Skip .NET Framework tests on non-Windows platforms #1842

Job	Run time
build (macos-latest, Release)	8m 13s
build (macos-latest, Debug)	4m 44s
build (windows-latest, Release)	11m 27s
build (ubuntu-latest, Release)	5m 16s
build (windows-latest, Debug)	10m 32s
build (ubuntu-latest, Debug)	5m 27s
publish-coverage / publish-coverage	35s
	46m 14s

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