CI: test against Julia nightly #592

Job	Run time
Documentation	15m 34s
test (1.6, stable, ubuntu-latest)	9m 27s
test (1.6, devel, ubuntu-latest)	10m 18s
test (1.6, 1.1, ubuntu-latest)	18m 57s
test (1.10, stable, ubuntu-latest)	9m 7s
test (1.10, devel, ubuntu-latest)	13m 7s
test (1.10, 1.1, ubuntu-latest)	12m 28s
test (1.11, stable, ubuntu-latest)	11m 28s
test (1.11, devel, ubuntu-latest)	19m 32s
test (1.11, 1.1, ubuntu-latest)	21m 51s
test (nightly, stable, ubuntu-latest)	11m 4s
test (nightly, devel, ubuntu-latest)	18m 47s
	2h 51m 40s

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