ci: fix linux packages cache

.github/workflows/linux-packages.yml

@@ -71,10 +71,13 @@ jobs:
           sudo apt-get update
           sudo apt-get install -y libwebkit2gtk-4.1-dev libgtk-3-dev libayatana-appindicator3-dev librsvg2-dev libxdo-dev pkg-config unzip xdg-utils
 
+      # `workspaces`: rust-cache v2 has no `workdir` input, and with it the
+      # cache would look for Cargo.toml at the repository root and save nothing.
       - name: Rust cache
         uses: swatinem/rust-cache@6323deb102c322ba6fcbdcafc7e3dddab59af2b6 #v2.9.2
         with:
-          workdir: ./src-tauri
+          workspaces: "src-tauri -> target"
+          save-if: ${{ github.ref == 'refs/heads/main' }}
 
       - name: Install frontend dependencies
         run: pnpm install --frozen-lockfile