ci: avoid downloading the repository twice

Avoid downloading the repository twice by leveraging the repository
already installed by the actions/checkout GitHub Action, instead of
downloading it again from GitHub.

.github/workflows/check.yml

@@ -21,12 +21,7 @@ jobs:
 
       - id: get-derivations
         run: |
-          nix flake show --json \
-            github:${{
-              github.repository
-            }}/${{
-              github.event.pull_request.head.sha || github.sha
-            }} |
+          nix flake show --json |
             jq \
               --argjson changed_files "$(
                 if ${{ github.event_name == 'pull_request' }}; then