Add annotation to untyped function causing mypy note

[huonw]

Running pants check :: currently gets a note level warning from mypy. This PR fixes it.

src/python/pants/core/util_rules/external_tool_test.py: note: In function "test_export":
src/python/pants/core/util_rules/external_tool_test.py:194:5: note: By default the bodies of untyped functions are not checked, consider using --check-untyped-defs  [annotation-unchecked]
src/python/pants/core/util_rules/external_tool_test.py:215:5: note: By default the bodies of untyped functions are not checked, consider using --check-untyped-defs  [annotation-unchecked]

This appears to be caused by a minor oversight in a test in #21258 (missing the -> None).