ci: cache cargo registry and target between Flatpak builds

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018jkHZhr7ZzHGGuE2bJQmGa

.gitea/workflows/build-flatpak.yml

@@ -106,7 +106,7 @@ jobs:
           curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --profile minimal
           echo "$HOME/.cargo/bin" >> "$GITHUB_PATH"
 
-      - name: Build agent from upstream source
+      - name: Fetch upstream source
         if: steps.check.outputs.build == 'true'
         env:
           TOKEN: ${{ secrets.HeiniRemoteFlatpakActionReleases }}
@@ -117,7 +117,23 @@ jobs:
           git -C upstream remote add origin "$UPSTREAM_URL"
           git -C upstream fetch -q --depth 1 origin "${{ steps.ver.outputs.ref }}"
           git -C upstream checkout -q FETCH_HEAD
-          cargo build --release -p heiniremote-agent --manifest-path upstream/Cargo.toml
+
+      # Same cache layout as the upstream workflows: without it every build
+      # compiled all dependencies from scratch (the bulk of the job's time).
+      - name: Cache cargo
+        if: steps.check.outputs.build == 'true'
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/.cargo/registry
+            ~/.cargo/git
+            upstream/target
+          key: cargo-flatpak-${{ hashFiles('upstream/Cargo.lock') }}
+          restore-keys: cargo-flatpak-
+
+      - name: Build agent from upstream source
+        if: steps.check.outputs.build == 'true'
+        run: cargo build --release -p heiniremote-agent --manifest-path upstream/Cargo.toml
 
       - name: Install Flatpak runtime and SDK
         if: steps.check.outputs.build == 'true'