fix(ci): use Corepack for pnpm setup when packageManager field is set

pnpm/action-setup@v4 errors with "Multiple versions of pnpm specified"
even when no explicit version input is provided, if the repo has a
packageManager field in package.json.

Switch to Corepack for repos that pin their pnpm version via the
packageManager field. Corepack reads the version from package.json
directly and installs it without conflicting with pnpm/action-setup.

Repos without a packageManager field continue using pnpm/action-setup@v4
with version: latest (unchanged behavior).

Unblocks headlamp-polaris-plugin PR #103 (ci/pin-pnpm-version).

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.github/workflows/plugin-ci.yaml

@@ -24,8 +24,8 @@ jobs:
           if [ -f "pnpm-lock.yaml" ]; then
             echo "manager=pnpm" >> $GITHUB_OUTPUT
             # Check for packageManager field in package.json (Corepack pinning).
-            # pnpm/action-setup@v4 errors if both `version` input and `packageManager`
-            # are set, so we detect here and use two conditional steps below.
+            # pnpm/action-setup@v4 errors when `packageManager` is set (even without
+            # a `version` input), so we use Corepack directly for those repos.
             PM=$(node -e "try{const p=require('./package.json');const v=p.packageManager||'';console.log(v.startsWith('pnpm@')?'true':'false')}catch(e){console.log('false')}" 2>/dev/null || echo "false")
             echo "has_package_manager=$PM" >> $GITHUB_OUTPUT
           else
@@ -33,11 +33,11 @@ jobs:
             echo "has_package_manager=false" >> $GITHUB_OUTPUT
           fi
 
-      - name: Setup pnpm (version from packageManager field)
+      - name: Setup pnpm (via Corepack, reads version from packageManager field)
         if: steps.pkg-manager.outputs.manager == 'pnpm' && steps.pkg-manager.outputs.has_package_manager == 'true'
-        uses: pnpm/action-setup@v4
-        with:
-          run_install: false
+        run: |
+          corepack enable pnpm
+          corepack install
 
       - name: Setup pnpm (version latest)
         if: steps.pkg-manager.outputs.manager == 'pnpm' && steps.pkg-manager.outputs.has_package_manager == 'false'