Only run PR checks on Ubuntu by default #4713

Job	Run time
Resolve environment (windows-latest, default)	24s
Resolve environment (ubuntu-latest, default)	11s
Resolve environment (ubuntu-latest, linked)	15s
Resolve environment (windows-latest, nightly-latest)	52s
Resolve environment (macos-latest, linked)	15s
Resolve environment (macos-latest, default)	14s
Resolve environment (windows-latest, linked)	26s
Resolve environment (macos-latest, nightly-latest)	12s
Resolve environment (ubuntu-latest, nightly-latest)	10s
Resolve environment (ubuntu-latest, default)	32s
Resolve environment (windows-latest, default)	1m 27s
Resolve environment (windows-latest, linked)	1m 25s
Resolve environment (macos-latest, default)	1m 39s
Resolve environment (macos-latest, nightly-latest)	1m 56s
Resolve environment (ubuntu-latest, nightly-latest)	38s
Resolve environment (windows-latest, nightly-latest)	2m 33s
Resolve environment (macos-latest, linked)	1m 51s
Resolve environment (ubuntu-latest, linked)	32s
	15m 32s

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