Use 12.x branch for love-appimage-source Linux build.

.github/workflows/main.yml

@@ -21,6 +21,7 @@ jobs:
       uses: actions/checkout@v2
       with:
         repository: MikuAuahDark/love-appimage-source
+        ref: 12.x
     - name: Checkout LÖVE
       uses: actions/checkout@v2
       with: