substitute: Fix "No such file or directory" message on Darwin

On Darwin, "chmod -f" does not suppress an error message if the file
doesn't exist. So just check if the file exists.

pkgs/stdenv/generic/setup.sh

@@ -370,7 +370,7 @@ substitute() {
         content="${content//"$pattern"/$replacement}"
     done
 
-    chmod -f +w "$output" || true
+    if [ -e "$output" ]; then chmod +w "$output".tmp; fi
     printf "%s" "$content" > "$output"
 }