CI: Update AppImage build workflow to avoid using actions/checkout.

.github/workflows/build.yml

@@ -57,7 +57,6 @@ jobs:
       image: ghcr.io/mu-editor/mu-appimage:2022.05.01
     name: Build AppImage
     steps:
-      - uses: actions/checkout@v4
       - name: Display system info
         run: |
           uname -a
@@ -68,13 +67,21 @@ jobs:
           python -m pip --version
           pip --version
           pip list --verbose
+      - name: Clone Mu
+        run: |
+            git init
+            git remote add origin ${{ github.server_url }}/${{ github.repository }}.git
+            git fetch --progress --depth=100 origin ${{ github.sha }}
+            git checkout --progress FETCH_HEAD
+            echo "cd ~/mu" > ~/.bashrc_new && cat ~/.bashrc >> ~/.bashrc_new
+            rm ~/.bashrc && mv ~/.bashrc_new ~/.bashrc
       - name: Install Mu test dependencies
         run: |
           pip install .[tests]
           pip list
       - run: mkdir upload
       - name: Build Linux AppImage
-        run: xvfb-run make linux
+        run: QT_QPA_PLATFORM=offscreen make linux
       # GitHub actions upload artifact breaks permissions, workaround using tar
       # https://github.com/actions/upload-artifact/issues/38
       - name: Tar AppImage to maintain permissions