The dependency also needs to go into deps.bzl so run_tests.bzl works.

Author: helly25

toolchain/deps.bzl

@@ -36,3 +36,11 @@ def bazel_toolchain_dependencies():
         )
 
     # Skip bazel_skylib_workspace because we are not using lib/unittest.bzl
+
+    if not native.existing_rule("helly25_bzl"):
+        http_archive(
+            name = "helly25_bzl",
+            strip_prefix = "bzl-0.1.2",
+            url = "https://github.com/helly25/bzl/releases/download/0.1.2/bzl-0.1.2.tar.gz",
+            sha256 = "404f8473bcaad2e370752e57d274d2093eb87ca94cb9a597c1a3553b76743206",
+        )