Make Github action build a specific variant.

.github/workflows/docker.yml

@@ -15,4 +15,4 @@ jobs:
       run: cd reproducible-builds && docker build -t signal-android . && cd ..
 
     - name: Test build
-      run: docker run --rm -v $(pwd):/project -w /project signal-android ./gradlew clean assembleRelease
+      run: docker run --rm -v $(pwd):/project -w /project signal-android ./gradlew clean assemblePlayProdRelease