feat: add nightly download button

I have no idea if it will work, but hopefully it does!

.github/workflows/android.yml

@@ -23,4 +23,9 @@ jobs:
     - name: Grant execute permission for gradlew
       run: chmod +x gradlew
     - name: Build with Gradle
-      run: ./gradlew buildNightly
+      run: ./gradlew assembleNightly
+    - name: Upload a Build Artifact
+    - uses: actions/upload-artifact@v3.1.2
+      with:
+       name: moshidon-nightly
+       path: ./mastodon/build/outputs/apk/nightly/moshidon-nightly.apk