fix(ci): post-runner-image cleanup (#7)

## Summary
Three follow-up fixes uncovered by the first end-to-end CI run on the self-hosted act_runner image (smoke-test PR).

- **`check` job** — replace `nrwl/nx-set-shas@v4` (GitHub-API only, 404 on Gitea) with a manual shell step that derives `NX_BASE`/`NX_HEAD` from local git history (merge-base on `pull_request`, `HEAD~1` on `push`).
- **`perf` job** — pin to `catthehacker/ubuntu:full-22.04` so Lighthouse CI finds a real Chrome. The default act image (`act-22.04`) is the minimal variant, ships without browsers.
- **`package.json`** — declare `pnpm.onlyBuiltDependencies` to silence the "Ignored build scripts" warning and approve only the post-install hooks we actually rely on (Nx, Prisma, esbuild, swc, native watchers/resolvers).

No ADR change — these are purely operational adjustments.

## Test plan
- [ ] CI run on this PR: `check` and `perf` jobs both green.
- [ ] No "Ignored build scripts" warning in `pnpm install` step output.
- [ ] After merge, `push` event on `main` re-runs `check`, `scan`, `perf`, `a11y` cleanly (no `nx-set-shas` 404).

---------

Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #7

.gitea/workflows/ci.yml

@@ -18,7 +18,25 @@ jobs:
       - uses: actions/checkout@v4
         with:
           fetch-depth: 0
-      - uses: nrwl/nx-set-shas@v4
+      # Derive NX_BASE / NX_HEAD for `nx affected`. Replaces
+      # nrwl/nx-set-shas@v4, which is GitHub-only (it queries the GitHub
+      # API to find the last successful workflow run, returning 404 on
+      # Gitea). HEAD~1 is a reasonable approximation for push events on
+      # a squash-merge trunk; pull_request uses the merge-base with the
+      # target branch.
+      - name: Derive Nx affected base and head
+        shell: bash
+        run: |
+          # `actions/checkout@v4` with fetch-depth: 0 already pulls every
+          # branch and tag, so origin/<base_ref> is present locally — no
+          # extra `git fetch` is needed (and `--depth=0` is invalid: git
+          # requires a positive integer).
+          if [ "${{ github.event_name }}" = "pull_request" ]; then
+            echo "NX_BASE=$(git merge-base HEAD origin/${{ github.base_ref }})" >> "$GITHUB_ENV"
+          else
+            echo "NX_BASE=HEAD~1" >> "$GITHUB_ENV"
+          fi
+          echo "NX_HEAD=HEAD" >> "$GITHUB_ENV"
       - uses: pnpm/action-setup@v3
       - uses: actions/setup-node@v4
         with:
@@ -70,6 +88,12 @@ jobs:
 
   perf:
     runs-on: [self-hosted, on-prem]
+    # Lighthouse CI drives a real Chrome instance; the default act runner
+    # image (catthehacker/ubuntu:act-22.04) ships without one. The :full
+    # variant adds Chrome, Firefox, and the GUI-test toolchain — pinned
+    # to the same Ubuntu 22.04 base as the default labels for parity.
+    container:
+      image: catthehacker/ubuntu:full-22.04
     steps:
       - uses: actions/checkout@v4
       - uses: pnpm/action-setup@v3