chore: do not run functional tests on pull-requests forks

These tests fail because they need the GITGUARDIAN_API_KEY variable, but
it's not available for pull-requests coming from forks.

Fixes GitGuardian#374

.github/workflows/main.yml

@@ -113,6 +113,10 @@ jobs:
           fail_ci_if_error: false
 
       - name: Run functional tests
+        # For security reasons, secrets are not available when running tests for a pull request from a fork. This causes
+        # all functional tests to fail because GITGUARDIAN_API_KEY is not defined. To avoid this, skip this step if we
+        # were triggered by a pull request from a fork.
+        if: github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name == github.repository
         shell: bash
         run: |
           make functest GITGUARDIAN_API_KEY=${{ secrets.GITGUARDIAN_API_KEY }} GITGUARDIAN_API_URL=${{ secrets.GITGUARDIAN_API_URL }}