fix(e2e): normalize reusable workflow reference to @main

Replace the branch-based reference with @main for the plugin-e2e
reusable workflow. The @main branch of .github is protected and
already contains all necessary pnpm support. This removes the drift
risk from referencing a non-protected branch.

Refs: PRI-1001

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

.github/workflows/e2e.yaml

@@ -16,7 +16,7 @@ concurrency:
 
 jobs:
   e2e:
-    uses: privilegedescalation/.github/.github/workflows/plugin-e2e.yaml@hugh/add-pnpm-support-plugin-e2e
+    uses: privilegedescalation/.github/.github/workflows/plugin-e2e.yaml@main
     with:
       node-version: '22'
       headlamp-version: v0.40.1