ci: point test-harness submodule to feat/add-env-var-tag branch

Temporary: CI needs the @env-var tag from flagd-testbed#359.
Revert to released branch once that PR is merged and tagged.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: aepfli

.gitmodules

@@ -4,7 +4,7 @@
 [submodule "providers/flagd/test-harness"]
 	path = providers/flagd/test-harness
 	url = https://github.com/open-feature/test-harness.git
-	branch = v3.0.1
+	branch = feat/add-env-var-tag
 [submodule "providers/flagd/spec"]
 	path = providers/flagd/spec
 	url = https://github.com/open-feature/spec.git