ci(lighthouse): drop concurrency block and dependency caching

Address review feedback from @mydea: this nightly cron does not need
either concurrency control or a shared dependency cache.

- The concurrency group keyed on `github.run_id` was a no-op (each run
  has a unique id, so nothing ever collided) and only obscured intent.
- The `CACHED_DEPENDENCY_PATHS` env block existed solely to feed the
  `install-dependencies` composite action's cache step. Replacing that
  action with a plain `yarn install --frozen-lockfile --ignore-engines`
  removes the cache plumbing for what is at most a ~30s saving on a
  workflow that runs once per day.

Co-Authored-By: claude-sonnet-4-5 <noreply@anthropic.com>

Author: HazAT

.github/workflows/lighthouse.yml

@@ -8,16 +8,6 @@ on:
     - cron: '0 0 * * *'
   workflow_dispatch:
 
-concurrency:
-  group: lighthouse-${{ github.run_id }}
-  cancel-in-progress: false
-
-env:
-  CACHED_DEPENDENCY_PATHS: |
-    ${{ github.workspace }}/node_modules
-    ${{ github.workspace }}/packages/*/node_modules
-    ${{ github.workspace }}/dev-packages/*/node_modules
-
 jobs:
   build-and-upload:
     name: Bundle and upload Lighthouse cells
@@ -35,7 +25,7 @@ jobs:
       - uses: actions/setup-node@v6
         with:
           node-version-file: 'package.json'
-      - uses: ./.github/actions/install-dependencies
+      - run: yarn install --frozen-lockfile --ignore-engines
       - run: yarn build:ci
       - run: yarn build:tarball
       - run: yarn test:prepare