ci: add libsdl3-dev for Ubuntu runner

Add the libsdl3 backport PPA for the Ubuntu CI runner because the latest
Ubuntu image does not have it.

Signed-off-by: Rafael Kitover <rkitover@gmail.com>

.github/workflows/ubuntu-build.yml

@@ -44,7 +44,10 @@ jobs:
 
       - name: Install Dependencies
         run: >-
-          bash installdeps; if [ "${{ matrix.build_compiler }}" = clang ]; then sudo apt -y install clang; fi
+          sudo add-apt-repository ppa:hrzhu/sdl3-backport
+          sudo apt update
+          bash installdeps
+          if [ "${{ matrix.build_compiler }}" = clang ]; then sudo apt -y install clang; fi
 
       - name: Install xvfb
         run: sudo apt -y install xvfb