Revert "Add `riscv64` support for flake.nix" #587

Job	Run time
Build windows executable	40s
Build tarballs	28s
Build static musl binaries (amd64)	30s
Build static musl binaries (i386)	36s
Build static musl binaries (ppc64le)	3m 7s
Build static musl binaries (arm64v8)	3m 21s
Build static musl binaries (arm32v7)	3m 12s
Build static musl binaries (s390x)	3m 23s
Build static musl binaries (riscv64)	3m 13s
Test windows binaries	15s
Publish tarballs & binaries	0s
	18m 45s

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