Prune idle sessions before starting new ones #651

Job	Run time
build (ubuntu-latest, Release)	3m 7s
build (windows-latest, Debug)	5m 0s
build (ubuntu-latest, Debug)	3m 13s
build (windows-latest, Release)	4m 58s
build (macos-latest, Release)	3m 31s
build (macos-latest, Debug)	2m 7s
publish-coverage	-1s
	21m 55s

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