Allow "create-dmg" to be in other places

If installed with homebrew, create-dmg will be installed at a different
location. It makes more sense to use the 'which' utility to find where
it is.

install/macos/build-app.py

@@ -88,7 +88,7 @@ def sign_app_bundle(build_path, dist_path, app_path):
     print(f"○ Signed app bundle: {app_path}")
 
     # Detect if create-dmg is installed
-    if not os.path.exists("/usr/local/bin/create-dmg"):
+    if not shutil.which("create-dmg"):
         print("create-dmg is not installed, skipping creating a DMG")
         return