stdenv substitute: avoid using a temporary file

- IMO using a temporary is not needed here (anymore),
- temporary at that location can cause a problem (in a specific case):
for example, when using the substituteAll function from nixpkgs
on a single file directly under /nix/store/ (or ./foo-file),
the stdenv's substitute tries to create a temporary directly under
/nix/store, which causes problems on chrooted darwin
(according to @copumpkin earlier today on IRC)

pkgs/stdenv/generic/setup.sh

@@ -370,9 +370,8 @@ substitute() {
         content="${content//"$pattern"/$replacement}"
     done
 
-    printf "%s" "$content" > "$output".tmp
-    if [ -x "$output" ]; then chmod +x "$output".tmp; fi
-    mv -f "$output".tmp "$output"
+    chmod -f +w "$output" || true
+    printf "%s" "$content" > "$output"
 }