Fix UnboundLocalError for asset_url on Piper install (macOS)

software/source/server/services/tts/piper/tts.py

@@ -57,8 +57,10 @@ class Tts:
             PIPER_ASSETNAME = f"piper_{OS}_{ARCH}.tar.gz"
             PIPER_URL = "https://github.com/rhasspy/piper/releases/latest/download/"
 
+            asset_url = f"{PIPER_URL}{PIPER_ASSETNAME}"
+
             if OS == "windows":
-                asset_url = f"{PIPER_URL}{PIPER_ASSETNAME}".replace(".tar.gz", ".zip")
+                asset_url = asset_url.replace(".tar.gz", ".zip")
 
             # Download and extract Piper
             urllib.request.urlretrieve(asset_url, os.path.join(PIPER_FOLDER_PATH, PIPER_ASSETNAME))