Add `riscv64` support for flake.nix #572

Job	Run time
Build tarballs	27s
Build windows executable	43s
Build static musl binaries (amd64)	33s
Build static musl binaries (i386)	40s
Build static musl binaries (ppc64le)	3m 10s
Build static musl binaries (arm64v8)	3m 16s
Build static musl binaries (arm32v7)	3m 12s
Build static musl binaries (s390x)	3m 8s
Build static musl binaries (riscv64)	3m 12s
Test windows binaries	12s
Publish tarballs & binaries	0s
	18m 33s

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