Moves scripts and CI/CD to nextest #484

Job	Run time
clippy/stable	19s
Pass MSRV values to other jobs	2s
cross_test/s390x-unknown-linux-gnu/stable	7m 35s
miri	12m 13s
nostd/thumbv6m-none-eabi/stable	18s
format/nightly	15s
cargo-careful	53s
cross_test/i686-unknown-linux-gnu/stable	1m 32s
7701 docs/nightly	23s
tests/stable	2m 13s
tests/beta	2m 12s
tests/nightly	2m 24s
blas-msrv	40s
tests/1.64.0	2m 51s
conclusion	3s
	33m 53s

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