Simplify actions matrix. (#31) #42

Job	Run time
build (ubuntu-latest, Release, 11)	10s
build (ubuntu-latest, Release, 20)	12s
build (ubuntu-latest, Debug, 11)	13s
build (ubuntu-latest, Debug, 20)	12s
build (ubuntu-20.04, Release, 11)	14s
build (ubuntu-20.04, Release, 20)	11s
build (ubuntu-20.04, Debug, 11)	14s
build (ubuntu-20.04, Debug, 20)	16s
build (macos-latest, Release, 11)	16s
build (macos-latest, Release, 20)	20s
build (macos-latest, Debug, 11)	15s
build (macos-latest, Debug, 20)	16s
build (macos-12, Release, 11)	27s
build (macos-12, Release, 20)	25s
build (macos-12, Debug, 11)	23s
build (macos-12, Debug, 20)	29s
build (windows-latest, Release, 11)	49s
build (windows-latest, Release, 20)	1m 16s
build (windows-latest, Debug, 11)	54s
build (windows-latest, Debug, 20)	55s
build (windows-2019, Release, 11)	1m 28s
build (windows-2019, Release, 20)	1m 53s
build (windows-2019, Debug, 11)	1m 17s
build (windows-2019, Debug, 20)	1m 23s
	14m 28s

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