ci: Remove +nightly and use default #876

Job	Run time
fmt	6s
msrv (ubuntu-latest)	1m 1s
clippy	8s
cross (ubuntu-latest, nightly)	2m 28s
msrv (windows-latest)	1m 3s
cross (ubuntu-latest, stable)	1m 28s
security_audit	7s
cross (macos-latest, nightly)	41s
cross (macos-latest, stable)	25s
test (ubuntu-latest, nightly)	1m 44s
wine	1m 30s
test (ubuntu-latest, beta)	1m 42s
test (ubuntu-latest, stable)	1m 34s
test (windows-latest, nightly)	1m 45s
test (windows-latest, beta)	1m 47s
test (windows-latest, stable)	3m 9s
test (macos-latest, nightly)	1m 35s
test (macos-latest, beta)	1m 47s
test (macos-latest, stable)	1m 19s
test (windows-latest, nightly-x86_64-pc-windows-gnu)	2m 21s
test (windows-latest, nightly-i686-pc-windows-msvc)	2m 9s
test (windows-latest, nightly-i686-pc-windows-gnu)	2m 42s
	32m 31s

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