Rust

nix: add cargo-llvm-cov to nix shell + niv update #5753

Job	Run time
Build (feature permutations)	3m 22s
Build (stable MSRV)	24s
Lints	1m 9s
Unit + Doc Tests	1m 42s
Integration Test (AArch64)	1m 13s
Integration Test (IA-32)	1m 42s
Integration Test (x86_64)	1m 34s
Integration Test (x86_64 Windows)	3m 21s
Build Standard Binary (nightly)	28s
Test Coverage	44s
Unit + Doc Tests (Miri)	1m 6s
Nightly (build, test, doc)	2m 34s
	19m 19s

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