ci: split build and test jobs, disable tests

The test suite is too heavy for the current CI infra (CPU/RAM
exhaustion), so it must not run alongside lint/build feedback.

- add a dedicated test job (needs: build) running runtime deps
  install, subuid/subgid setup and 'cargo test'
- share the compiled target/ dir and cargo caches between both jobs
  via actions/cache keyed on the commit sha, so the test job only
  compiles the test binaries
- disable the test job with 'if: false' until infra can handle it;
  re-enable by removing that line
- snap: bump upload-artifact v3 -> v4 (v3 is decommissioned) and
  checkout to v6
- update images to ubuntu:26.04

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
   build:
     runs-on: ubuntu-latest
     container:
-      image: ubuntu:24.04
+      image: ubuntu:26.04
       options: --privileged --cap-add SYS_ADMIN --security-opt apparmor:unconfined
     steps:
     - name: Set up container image
@@ -30,6 +30,16 @@ jobs:
       run: |
         sudo apt-get update
         sudo apt-get install -y pkg-config libssl-dev libgpg-error-dev libgpgme-dev
+    - name: Restore cargo/target cache
+      uses: actions/cache@v4
+      with:
+        path: |
+          ~/.cargo/registry
+          ~/.cargo/git
+          target
+        key: ci-build-${{ github.sha }}
+        restore-keys: |
+          ci-build-
     - name: Build
       run: cargo build
       env:
@@ -38,6 +48,37 @@ jobs:
       run: cargo clippy --all-targets --all-features
       env:
         RUSTFLAGS: -Dwarnings
+
+  test:
+    # Disabled: test suite is too heavy for current CI infra (CPU/RAM exhaustion).
+    # Re-enable by removing the `if: false` line below.
+    if: false
+    needs: build
+    runs-on: ubuntu-latest
+    container:
+      image: ubuntu:26.04
+      options: --privileged --cap-add SYS_ADMIN --security-opt apparmor:unconfined
+    steps:
+    - name: Set up container image
+      run: |
+        apt-get update
+        apt-get install -y nodejs sudo curl wget ca-certificates build-essential
+    - uses: actions/checkout@v6
+    - uses: dtolnay/rust-toolchain@stable
+    - name: Restore cargo/target cache
+      uses: actions/cache@v4
+      with:
+        path: |
+          ~/.cargo/registry
+          ~/.cargo/git
+          target
+        key: ci-build-${{ github.sha }}
+        restore-keys: |
+          ci-build-
+    - name: Install build dependencies
+      run: |
+        sudo apt-get update
+        sudo apt-get install -y pkg-config libssl-dev libgpg-error-dev libgpgme-dev
     - name: Install runtime system dependencies
       run: |
         sudo apt-get update
@@ -56,10 +97,10 @@ jobs:
     outputs:
       snap-file: ${{ steps.build-snap.outputs.snap }}
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - uses: snapcore/action-build@v1
         id: build-snap
-      - uses: actions/upload-artifact@v3
+      - uses: actions/upload-artifact@v4
         with:
           name: snap
           path: ${{ steps.build-snap.outputs.snap }}