Revert "ci: switch E2E workflow to ubuntu-latest runner"

This reverts commit dbc69fb. The Headlamp instance at headlamp.animaniacs.farh.net
is not publicly accessible, so E2E tests require the self-hosted k3s-animaniacs
runner with network access to the internal domain.

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Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

.github/workflows/e2e.yaml

@@ -9,7 +9,7 @@ on:
 
 jobs:
   e2e:
-    runs-on: ubuntu-latest
+    runs-on: k3s-animaniacs
     timeout-minutes: 15
 
     steps: