ci: pass current flint GitHub env vars

Signed-off-by: Gregor Zeitlinger <gregor.zeitlinger@grafana.com>

Author: zeitlinger

.github/workflows/reusable-link-check.yml

@@ -20,7 +20,9 @@ jobs:
         if: github.event_name == 'pull_request'
         env:
           GITHUB_TOKEN: ${{ github.token }}
-          GITHUB_HEAD_SHA: ${{ github.event.pull_request.head.sha }}
+          GITHUB_REPOSITORY: ${{ github.repository }}
+          GITHUB_BASE_REF: ${{ github.base_ref }}
+          GITHUB_HEAD_REF: ${{ github.head_ref }}
           PR_HEAD_REPO: ${{ github.event.pull_request.head.repo.full_name }}
         run: mise run lint