fix: tighten NOTE-marker detection in check-inspektor-gadget-pin.sh (#930)

The NOTE-marker check used `grep -qiE 'NOTE'`, an unanchored substring
match that would also match comment text containing "denote",
"annotate", "notebook", etc. -- not just a standalone NOTE: marker --
causing a false-positive CI failure on an otherwise-fine PR. Switch to
`grep -qiw 'NOTE'` for a word-boundary match. Mirrors the same fix
landing in armosec/private-node-agent.