nix: overwrite, don't append, to $RUSTFLAGS

This matches the behavior of the actual `nix build` more closely, and might also
help Anton, since he was debugging some recompilation issues on his machine,
where `RUSTFLAGS` might have become inconsistent due to VS Code.

Signed-off-by: Austin Seipp <aseipp@pobox.com>

flake.nix

@@ -161,7 +161,7 @@
           export ZSTD_SYS_USE_PKG_CONFIG=1
           export LIBSSH2_SYS_USE_PKG_CONFIG=1
         '' + pkgs.lib.optionalString useMoldLinker ''
-          export RUSTFLAGS="-C link-arg=-fuse-ld=mold $RUSTFLAGS"
+          export RUSTFLAGS="-C link-arg=-fuse-ld=mold"
         '';
       };
     }));