Revert "Add `riscv64` support for flake.nix" #588

Job	Run time
Build windows executable	46s
Build tarballs	26s
Build static musl binaries (amd64)	36s
Build static musl binaries (i386)	36s
Build static musl binaries (ppc64le)	3m 7s
Build static musl binaries (arm64v8)	3m 24s
Build static musl binaries (arm32v7)	3m 10s
Build static musl binaries (s390x)	3m 24s
Build static musl binaries (riscv64)	3m 6s
Test windows binaries	18s
Publish tarballs & binaries	0s
	18m 53s

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