fix: pipelines can have either trigger or script

They canno thave both, so here I add assertions support and use them to avoid this case.

tests/modules_test.nix

@@ -98,6 +98,59 @@
             assert_file_contains ${package} 'export EXAMPLE="/nix/store/.*-hello-.*"'
           '';
       }
+      {
+        name = "trigger job without script is valid";
+        expected = {
+          trigger = "some/project";
+          stage = "test";
+          image = "$NIX_CI_IMAGE";
+        };
+        actual =
+          (cilib.mkCI {
+            pipelines."test" = {
+              stages = ["test"];
+              jobs."trigger_job" = {
+                stage = "test";
+                trigger = "some/project";
+                nix.enable = false;
+              };
+            };
+          }).pipelines."test".finalConfig."trigger_job";
+      }
+      {
+        name = "script job without trigger is valid";
+        expected = {
+          stage = "test";
+          image = "$NIX_CI_IMAGE";
+          before_script = ["source setup_nix_ci \"gitlab-ci:pipeline:test:job-deps:test\""];
+          script = ["echo hello"];
+          after_script = ["finalize_nix_ci"];
+        };
+        actual =
+          (cilib.mkCI {
+            pipelines."test" = {
+              stages = ["test"];
+              jobs."test" = {
+                stage = "test";
+                script = ["echo hello"];
+              };
+            };
+          }).pipelines."test".finalConfig."test";
+      }
+      {
+        name = "job cannot have both script and trigger";
+        expected = false;
+        actual = (builtins.tryEval (cilib.mkCI {
+          pipelines."test" = {
+            stages = ["test"];
+            jobs."bad" = {
+              stage = "test";
+              script = ["echo bad"];
+              trigger = "some/project";
+            };
+          };
+        })).success;
+      }
     ];
   };
 }