Fix CAPK required test name

Since the unit test in the cluster-api-provider-kubevirt is running with
github action matrix, the existing `unit_test` name for the required
test is wrong, and should be `unit_test (ubuntu-latest)`.

Signed-off-by: Nahshon Unna-Tsameret <nunnatsa@redhat.com>

config/prow/config.yaml

@@ -526,7 +526,7 @@ branch-protection:
         cluster-api-provider-kubevirt:
           required_status_checks:
             contexts:
-            - unit_test
+            - "unit_test (ubuntu-latest)"
             - integration
             - go-linter
             - check-gen