GitHubCI: add dotnet-only mac&win jobs #2548

Job	Run time
macOS--mono-only	6m 38s
macOS--dotnet6-and-mono	6m 52s
windows--legacyFramework-only	13m 32s
macOS--dotnet6-only	5m 28s
windows--dotnet6-only	8m 30s
linux22-github--dotnet-and-mono	4m 25s
linux22-github--dotnet-and-newmono	4m 3s
linux22-vanilla--stockmono-only	6m 4s
windows--dotnet6-and-legacyFramework	7m 9s
linux20-github--dotnet-and-newmono	4m 14s
linux20-vanilla--newmono-only	8m 32s
linux20-github--dotnet-and-mono	3m 46s
linux20-vanilla--stockmono-only	6m 2s
linux22-vanilla--newmono-only	6m 43s
linux22-vanilla--stockdotnet6-and-newmono	6m 51s
linux22-vanilla--stockdotnet6-and-stockmono	5m 30s
linux22-vanilla--stockdotnet6-only	6m 26s
conventions	1m 20s
snap_pkg	6m 6s
	1h 58m 11s

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