CI: Add linux build dependencies for Linux 'test bins' job

Need to fix a compilation error in CI which is
resulting from missing X11-related headers.

Some of these deps presumably have been removed or changed in the recent
"ubuntu-latest means Ubuntu 24.04" change.

See this tracking issue about the runner migration for details, maybe:
github[dot]com/actions[slash]runner-images/issues/10636

So, adding all of these deps just to be sure our code can compile
and be tested successfully in CI. These are the same deps as used
above in the main "build" Linux part of the workflow file.

.github/workflows/build.yml

@@ -238,6 +238,10 @@ jobs:
     needs: build
 
     steps:
+    - name: Install build dependencies - Linux
+      if: ${{ runner.os == 'Linux' }}
+      run: sudo apt-get update && sudo apt-get install -y git python3 python3-pip make gcc g++ libx11-dev libxkbfile-dev pkg-config libsecret-1-dev rpm xvfb ffmpeg zstd wget squashfs-tools
+
     - name: Checkout the latest code
       uses: actions/checkout@v4