test: fix the reference to the chromium sysroot repo under bzlmod

A temporary workaround; see bazelbuild/bazel#17141

tests/MODULE.bazel

@@ -161,7 +161,18 @@ llvm.toolchain(
         "linux-x86_64": "chromium_x64_sysroot",
     },
     sysroot_label_map = {
-        "@org_chromium_sysroot_linux_x64//:sysroot": "chromium_x64_sysroot",
+        # NOTE: canonical repo reference here!
+        #
+        # `org_chromium_sysroot_linux_x64` comes from `http_archive` which
+        # is currently defined in `WORKSPACE.bzlmod`.
+        #
+        # With `use_repo_rule` (Bazel 7) it'll be easy to move the
+        # `http_archive` that fetches this repo to `MODULE.bazel` but for now
+        # it requires some gymnastics; see this issue for context:
+        # https://github.com/bazelbuild/bazel/issues/17141#issuecomment-1622343142
+        #
+        # So, for now, we reference this http_archive by it's canonical name:
+        "@@org_chromium_sysroot_linux_x64//:sysroot": "chromium_x64_sysroot",
     },
     # We can share the downloaded LLVM distribution with the first configuration.
     toolchain_roots = {