chore: add test and docs for handling nix store paths in global variables

tests/cilib_test.nix

@@ -163,6 +163,28 @@
             };
           }).finalConfig;
       }
+      {
+        # it doesn't make much sense to have any nix store path in variables, but we ignore it for global variables
+        name = "ignore store paths in global variables";
+        expected = {
+          variables = {
+            HELLO = "world";
+            CURL = toString pkgs.curl;
+          };
+        };
+        actual =
+          (mkPipeline {
+            name = "test";
+            nixConfig.enable = true;
+            pipeline = {
+              variables = {
+                HELLO = "world";
+                CURL = toString pkgs.curl;
+              };
+              jobs = {};
+            };
+          }).finalConfig;
+      }
     ];
   };
 }