CI: Increase free space in workflow

Recently we've been running out of free space in the ubuntu 20.04
environment resulting in test failures. This appears to be caused
by a change in the default available free space and not because of
any change in OpenZFS. Try and avoid this failure by applying a
suggested workaround which removes some unnecessary files.

https://github.com/actions/virtual-environments/issues/2840

Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #11826