Only run PR checks on Ubuntu by default #1472

Job	Run time
Upload debug artifacts after failure in analyze (stable-v2.20.3)	33s
Upload debug artifacts after failure in analyze (linked)	20s
Upload debug artifacts after failure in analyze (default)	19s
Upload debug artifacts after failure in analyze (nightly-latest)	16s
Download and check debug artifacts after failure in analyze	4s
Upload debug artifacts after failure in analyze (stable-v2.20.3)	1m 46s
Upload debug artifacts after failure in analyze (default)	4m 4s
Upload debug artifacts after failure in analyze (linked)	3m 51s
Upload debug artifacts after failure in analyze (nightly-latest)	2m 56s
Download and check debug artifacts after failure in analyze	10s
	14m 19s

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