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

package.json

@@ -16,6 +16,22 @@
       "prettier --write"
     ]
   },
+  "pnpm": {
+    "onlyBuiltDependencies": [
+      "@nestjs/core",
+      "@parcel/watcher",
+      "@prisma/client",
+      "@prisma/engines",
+      "@swc/core",
+      "esbuild",
+      "less",
+      "lmdb",
+      "msgpackr-extract",
+      "nx",
+      "prisma",
+      "unrs-resolver"
+    ]
+  },
   "devDependencies": {
     "@analogjs/vite-plugin-angular": "~2.1.2",
     "@analogjs/vitest-angular": "~2.1.2",