Use the CI scripts from GZDoom as we have nasty dependencies

.github/workflows/appimage.yml

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+name: Continuous Integration
+
+on: [push, pull_request]
+
+jobs:
+  build-appimage:
+    name: Linux Clang 12 | AppImage
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v3
+      - name: Install VKDoom dependencies
+        run: |
+          sudo apt-get update
+          sudo apt-get install clang-12 libsdl2-dev libvpx-dev libwebp-dev cmake -y
+      - name: Install appimage-builder dependencies
+        run: |
+          sudo apt-get install binutils coreutils desktop-file-utils fakeroot fuse libgdk-pixbuf2.0-dev patchelf -y 
+          sudo apt-get install python3-pip python3-setuptools squashfs-tools strace util-linux zsync -y 
+          sudo apt-get install imagemagick-6.q16hdri -y
+      - name: Install appimage-builder
+        run: |
+          python3 -m pip install --upgrade pip
+          pip install appimage-builder
+      - name: Build zmusic dependency
+        run: |
+          git clone -b 1.1.12 https://github.com/ZDoom/ZMusic.git zmusic_build
+          mkdir ./zmusic_build/build
+          cd ./zmusic_build/build
+          cmake -DCMAKE_BUILD_TYPE=Release ..
+          cmake --build .
+          cd ../../
+          mkdir -p ./AppDir/zmusic/lib
+          mkdir ./AppDir/zmusic/include
+          mv ./zmusic_build/build/source/libzmusic* ./AppDir/zmusic/lib/
+          mv ./zmusic_build/include/zmusic.h ./AppDir/zmusic/include/
+      - name: Configure
+        run: cmake -B AppDir -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=/usr -DCMAKE_PREFIX_PATH=`pwd`/AppDir/zmusic -DPK3_QUIET_ZIPDIR=ON -DCMAKE_C_COMPILER=clang-12 -DCMAKE_CXX_COMPILER=clang++-12 .
+      - name: Build
+        shell: bash
+        run: |
+          export MAKEFLAGS=--keep-going
+          cmake --build AppDir --config Release --parallel `nproc`
+          cp ./AppDir/zmusic/lib/* ./AppDir/
+          mkdir -p ./AppDir/usr/share/icons
+          convert ./src/win32/icon1.ico[0] -flatten ./AppDir/usr/share/icons/game_icon.png
+      - name: Build AppImage
+        run: |
+          appimage-builder --skip-tests
+      - uses: actions/upload-artifact@v2
+        with:
+          name: AppImage
+          path: './*.AppImage*'