CI: Install zstd in Docker so that caches will match

Caches have a unique "version" that relates to the OS/compression
strategy.

zstd is the default for GitHub-hosted Linux runners.

For "self-hosted" or custom runners (or Docker containers, I guess?),
you have to ensure you have zstd on the system creating the cache,
or it won't be restorable in jobs running on GitHub-hosted Linux runners.

So.. Add zstd to the Docker Debian environment so we get compatible caches.

.github/workflows/build.yml

@@ -33,7 +33,7 @@ jobs:
     steps:
     - name: Install build dependencies - Linux
       if: ${{ runner.os == 'Linux' }}
-      run: apt-get update && apt-get install -y git python3 make gcc g++ libx11-dev libxkbfile-dev pkg-config libsecret-1-dev rpm xvfb ffmpeg
+      run: apt-get update && apt-get install -y git python3 make gcc g++ libx11-dev libxkbfile-dev pkg-config libsecret-1-dev rpm xvfb ffmpeg zstd
 
     - name: Checkout the latest code
       uses: actions/checkout@v3