Deploy production Linux builds to Steam (#47)

## Summary
- package the production Linux binary and Bevy assets under `build/linux`
- generate Steam TOTP credentials in CI
- deploy app 3409670 to the Steam `master` branch using the latest commit hash as the build description
- restrict packaging and deployment to pushes on `master`, excluding pull requests and manual runs

Reviewed-on: #47
Co-authored-by: Álex Román Núñez <eirexe@eirteam.moe>

.gitea/workflows/ci.yml

@@ -97,3 +97,31 @@ jobs:
       - name: Verify production binary is not Bevy/Rust dynamically linked
         run: |
           ! readelf -d target/production/shinespark | grep -E 'libbevy_dylib|libstd-.*\.so'
+
+      - name: Package Linux Steam depot
+        if: github.event_name == 'push' && github.ref == 'refs/heads/master'
+        run: |
+          mkdir -p build/linux
+          cp target/production/shinespark build/linux/shinespark
+          cp -R assets build/linux/assets
+          cp -R imported_assets build/linux/imported_assets
+
+      - if: github.event_name == 'push' && github.ref == 'refs/heads/master'
+        uses: CyberAndrii/steam-totp@v1
+        name: Generate Steam TOTP
+        id: steam-totp
+        with:
+          shared_secret: ${{ secrets.STEAM_SHARED_SECRET }}
+
+      - name: Deploy Linux build to Steam
+        if: github.event_name == 'push' && github.ref == 'refs/heads/master'
+        uses: game-ci/steam-deploy@v3
+        with:
+          username: ${{ secrets.STEAM_USERNAME }}
+          password: ${{ secrets.STEAM_PASSWORD }}
+          totp: ${{ steps.steam-totp.outputs.code }}
+          appId: 3409670
+          buildDescription: ${{ github.sha }}
+          rootPath: build
+          depot1Path: linux
+          releaseBranch: master