Fix regexp escaping (#132)

Escape `.` to `\\.`

Author: sinsukehlab

.github/workflows/1-copilot-extension.yml

@@ -58,7 +58,7 @@ jobs:
         uses: skills/action-check-file@v1
         with:
           file: ".devcontainer/devcontainer.json"
-          search: "GitHub.copilot"
+          search: "GitHub\\.copilot"
 
       # In README.md, switch step 1 for step 2.
       - name: Update to step 2