ci(e2e): Keep install-pnpm unchanged and install pnpm directly in E2E jobs

Only the E2E jobs drop the root node_modules, so only they need another way to
get pnpm. Canary, Lighthouse and local runs keep getting it through yarn.lock.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: mydea

.github/actions/install-pnpm/action.yml

@@ -1,16 +1,11 @@
 name: 'Install pnpm'
-description: 'Installs the pnpm version from the root devDependencies.'
+description: 'Puts the pnpm version from the root devDependencies on PATH. Run it after the dependencies are installed.'
 
 runs:
   using: 'composite'
   steps:
     # The dev dependency is the one pnpm version for CI and local runs. An app that needs another version sets
-    # `packageManager` in its package.json, and pnpm switches to that version. It is read from package.json rather
-    # than node_modules, so jobs that don't restore the monorepo's dependencies can use it.
-    - name: Resolve pnpm version
-      id: pnpm-version
+    # `packageManager` in its package.json, and pnpm switches to that version.
+    - name: Add pnpm to PATH
       shell: bash
-      run: echo "version=$(jq -er '.devDependencies.pnpm' "$GITHUB_WORKSPACE/package.json")" >> "$GITHUB_OUTPUT"
-    - uses: pnpm/action-setup@ea17c68df8912ef543352723c149a84f56e3d413 # v6.1.0
-      with:
-        version: ${{ steps.pnpm-version.outputs.version }}
+      run: echo "$GITHUB_WORKSPACE/node_modules/pnpm" >> "$GITHUB_PATH"

.github/workflows/build.yml

@@ -1088,8 +1088,14 @@ jobs:
         uses: denoland/setup-deno@v2.0.5
         with:
           deno-version: ${{ matrix.deno-version || 'v2.8.3' }}
+      # Not `install-pnpm`: it takes pnpm from the root node_modules, which this job doesn't restore.
+      - name: Resolve pnpm version
+        id: pnpm-version
+        run: echo "version=$(jq -er '.devDependencies.pnpm' package.json)" >> "$GITHUB_OUTPUT"
       - name: Install pnpm
-        uses: ./.github/actions/install-pnpm
+        uses: pnpm/action-setup@ea17c68df8912ef543352723c149a84f56e3d413 # v6.1.0
+        with:
+          version: ${{ steps.pnpm-version.outputs.version }}
 
       - name: Restore tarball artifacts
         uses: actions/download-artifact@v8
@@ -1242,8 +1248,14 @@ jobs:
         uses: denoland/setup-deno@v2.0.5
         with:
           deno-version: ${{ matrix.deno-version || 'v2.8.3' }}
+      # Not `install-pnpm`: it takes pnpm from the root node_modules, which this job doesn't restore.
+      - name: Resolve pnpm version
+        id: pnpm-version
+        run: echo "version=$(jq -er '.devDependencies.pnpm' package.json)" >> "$GITHUB_OUTPUT"
       - name: Install pnpm
-        uses: ./.github/actions/install-pnpm
+        uses: pnpm/action-setup@ea17c68df8912ef543352723c149a84f56e3d413 # v6.1.0
+        with:
+          version: ${{ steps.pnpm-version.outputs.version }}
 
       - name: Restore tarball artifacts
         uses: actions/download-artifact@v8