chore: switch action runtime from node20 to bun via composite action

Author: wadefletch

action.yml

@@ -19,5 +19,13 @@ inputs:
     default: "fixup[bot]"
 
 runs:
-  using: "node20"
-  main: "dist/index.js"
+  using: "composite"
+  steps:
+    - uses: oven-sh/setup-bun@v2
+    - run: bun ${{ github.action_path }}/dist/index.js
+      shell: bash
+      env:
+        INPUT_APP-ID: ${{ inputs.app-id }}
+        INPUT_PRIVATE-KEY: ${{ inputs.private-key }}
+        INPUT_FIXERS: ${{ inputs.fixers }}
+        INPUT_BOT-LOGIN: ${{ inputs.bot-login }}