stdenv: Substitute correctly chmods instead of .tmp which never exists

pkgs/stdenv/generic/setup.sh

···
         content="${content//"$pattern"/$replacement}"
     done
 
-    if [ -e "$output" ]; then chmod +w "$output".tmp; fi
+    if [ -e "$output" ]; then chmod +w "$output"; fi
     printf "%s" "$content" > "$output"
 }