Fix update-reference workflow by adding checkout step (#479)

The workflow was broken in commit 9bb3cde when the explicit checkout
step was replaced with the local ./.github/actions/setup action. Local
actions require the repository to be checked out first so the action
file can be found.

Author: jhrozek

.github/workflows/update-toolhive-reference.yml

@@ -25,6 +25,9 @@ jobs:
   update-reference:
     runs-on: ubuntu-latest
     steps:
+      - name: Checkout repository
+        uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6
+
       - name: Setup
         uses: ./.github/actions/setup