Moves scripts and CI/CD to nextest #483

Job	Run time
nostd/thumbv6m-none-eabi/stable	22s
clippy/stable	19s
Pass MSRV values to other jobs	4s
format/nightly	15s
cross_test/i686-unknown-linux-gnu/stable	1m 38s
miri	12m 35s
cross_test/s390x-unknown-linux-gnu/stable	7m 7s
cargo-careful	52s
docs/nightly	22s
blas-msrv	45s
tests/nightly	2m 28s
tests/1.64.0	3m 3s
tests/beta	1m 58s
tests/stable	2m 19s
conclusion	2s
	34m 9s

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