ci: checkout submodules in test job so gherkin tests run

The TestEvaluatorGherkin test depends on the test-harness submodule
and silently skips when it is absent. The test job did not check out
submodules, so the new gherkin tests never ran in CI.

Author: aepfli

.github/workflows/build.yaml

@@ -54,6 +54,8 @@ jobs:
     steps:
       - name: Checkout repository
         uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11 # v4
+        with:
+          submodules: recursive
       - name: Setup go
         uses: actions/setup-go@0c52d547c9bc32b1aa3301fd7a9cb496313a4491 # v5
         with: