We see `test_extension_store_with_test_extension` hang in untarring the
WASI SDK some times.

In lieu of trying to debug the problem, let's try shelling out for now
in the hope that the test becomes more reliable.

There's a bit of risk here because we're using async-tar for other
things (but probably not 300Mb tar files...)

Assisted-By: Zed AI

Closes #ISSUE

Release Notes:

- N/A