GitHubCI: add dotnet-only mac&win jobs #2545

Job	Run time
macOS--dotnet6-and-mono	8m 30s
macOS--dotnet6-only	5m 28s
macOS--mono-only	6m 24s
linux22-github	4m 55s
linux22-github--newmono	4m 22s
windows--dotnet6-and-legacyFramework	6m 25s
windows--legacyFramework-only	8m 12s
linux22-vanilla--stockdotnet6-and-newmono	6m 51s
linux22-vanilla--stockdotnet6-and-stockmono	7m 1s
linux22-vanilla--stockdotnet6-only	7m 3s
windows--dotnet6-only	8m 2s
linux20-github	3m 58s
linux20-github--newmono	5m 5s
linux20-vanilla--newmono-only	8m 34s
linux20-vanilla--stockmono-only	6m 46s
linux22-vanilla--newmono-only	8m 14s
linux22-vanilla--stockmono-only	5m 53s
conventions	1m 11s
snap_pkg	4m 55s
	1h 57m 49s

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