Only run PR checks on Ubuntu by default #7850

Job	Run time
Packaging: Action input (ubuntu-latest, default)	1m 5s
Packaging: Action input (ubuntu-latest, nightly-latest)	25s
Packaging: Action input (ubuntu-latest, linked)	31s
Packaging: Action input (ubuntu-latest, linked)	1m 26s
Packaging: Action input (ubuntu-latest, default)	1m 37s
Packaging: Action input (ubuntu-latest, nightly-latest)	1m 42s
	6m 46s

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