Revert "only use musl on x86_64"

This reverts commit 56f0f3dfa4.

This shouldn't be needed anymore since [this][0] reached nixos-unstable.

[0]: https://github.com/NixOS/nixpkgs/pull/242889

flake.nix

@@ -25,8 +25,8 @@
     let
       pkgs = nixpkgs.legacyPackages.${system};
 
-      # Use mold where possible
+      # Use mold on Linux
-      stdenv = if pkgs.stdenv.isLinux && pkgs.stdenv.isx86_64 then
+      stdenv = if pkgs.stdenv.isLinux then
         pkgs.stdenvAdapters.useMoldLinker pkgs.stdenv
       else
         pkgs.stdenv;