CI: Increase free space in workflow

[behlendorf]

Motivation and Context

In order to avoid some recent ubuntu 20.04 workflow failures caused
by a lack of space try and apply a suggested workaround. Alternately,
we could look in to further reducing the loopback device sizes if this
is still insufficient.

https://github.com/openzfs/zfs/pull/11822/checks?check_run_id=2239809026

Description

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.

actions/runner-images#2840

Note this appears to only be an issue for 20.04 and not 18.04.

How Has This Been Tested?

We'll see how it goes. This PR in intended to test if this resolves
the problem.

Types of changes

• GitHub actions CI changes
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.