Add NixOS packaging #280

Job	Run time
Clang Tidy	8s
Clippy nightly (optional)	31s
Build+test (stable, ubuntu-24.04)	1m 51s
Clippy	33s
Build+test (stable, ubuntu-22.04)	1m 45s
Check for documentation errors	41s
Format	12s
Build+test (beta, ubuntu-24.04)	1m 51s
Miri	1m 12s
Build+test (beta, ubuntu-22.04)	1m 42s
Build+test (nightly, ubuntu-24.04)	1m 50s
Packaging	2m 53s
Produce release packages (fedora, 40, rpm)	3m 31s
Build+test (nightly, ubuntu-22.04)	1m 39s
Produce release packages (ubuntu, 24.04, deb)	2m 13s
Valgrind	2m 12s
	24m 44s

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