CI: Move unstable testing to main branch. #24

Job	Run time
miri	2m 23s
build (ubuntu, stable, locked, true)	44m 37s
build (windows, stable, locked, false)	25m 42s
build (macos, stable, locked, false)	18m 11s
build (ubuntu, nightly, locked, false)	35m 3s
build (ubuntu, beta, locked, false)	38m 29s
build (ubuntu, stable, latest, false)	17m 25s
build (ubuntu, stable, minimal, false)	22m 52s
fuzz	17m 8s
deploy	0s
	3h 41m 50s

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