Only run PR checks on Ubuntu by default #6481

Job	Run time
autobuild-action (ubuntu-latest, linked)	11s
autobuild-action (windows-latest, linked)	34s
autobuild-action (macos-latest, linked)	14s
autobuild-action (ubuntu-latest, linked)	1m 41s
autobuild-action (macos-latest, linked)	2m 20s
autobuild-action (windows-latest, linked)	3m 40s
	8m 40s

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