nix-prefetch-darcs: fix tag JSON output error

Author: toastal

pkgs/build-support/fetchdarcs/nix-prefetch-darcs

@@ -164,7 +164,7 @@ cat <<EOF
 	"repository": $(json_escape "$repository"),
 EOF
 if [ -n "$tag" ]; then cat <<EOF
-	"tag": "$(json_escape "$tag")",
+	"tag": $(json_escape "$tag"),
 EOF
 elif [ -n "$darcs_hash" ]; then cat <<EOF
 	"darcs-hash": $(json_escape "$darcs_hash"),