replace hub command with gh (#10)

* replace hub with gh

ref: mislav/hub#3338 (comment)
ref: https://docs.github.com/ja/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally?tool=cli#modifying-an-active-pull-request-locally

* comment out the condition to test the action in unmerged branch

* Revert "comment out the condition to test the action in unmerged branch"

This reverts commit 6434288.

.github/workflows/test-action.yml

@@ -27,7 +27,7 @@ jobs:
           PR_URL="${{ github.event.issue.pull_request.url }}"
           PR_NUM=${PR_URL##*/}
           echo "Checking out from PR #$PR_NUM based on URL: $PR_URL"
-          hub pr checkout $PR_NUM
+          gh pr checkout $PR_NUM
 
       - name: Test Local Action
         id: test-action