ci: pin changesets/bot action to commit SHA for supply-chain security

Pin to master HEAD SHA as changesets/bot has no release tags.
Aligns with the repo's SHA-pinning policy (toss#325).

Author: guesung

.github/workflows/changeset-bot.yml

@@ -11,4 +11,4 @@ jobs:
   bot:
     runs-on: ubuntu-latest
     steps:
-      - uses: changesets/bot@main
+      - uses: changesets/bot@e755cedfffdc49f54ce5daa69d960a143941b787 # master