use the 12.x branch of megasource for CI builds

.github/workflows/main.yml

@@ -51,7 +51,7 @@ jobs:
       with:
         path: megasource
         repository: love2d/megasource
-        ref: master
+        ref: 12.x
     - name: Checkout
       uses: actions/checkout@v2
       with:

extra/appveyor/appveyor.yml

@@ -28,7 +28,7 @@ install:
 - for /D %%i in (*) do @if "%%i" NEQ "love" @move %%i love\%%i
 - for %%i in (*) do @if "%%i" NEQ "appveyor.yml" @move %%i love\%%i
 # clone megasource and move into top directory.
-- git clone --depth 1 https://github.com/love2d/megasource.git megasource
+- git clone --depth 1 --single-branch -branch 12.x https://github.com/love2d/megasource.git megasource
 - cd megasource
 - for /D %%i in (*) do @move %%i ..\%%i
 - for %%i in (*) do @move %%i ..\%%i