Legate driver tests: "no ucx" is not always the default

[manopapad]

Paraphrasing from a description of the issue by @bryevdv:

Most legate driver tests were originally developed in an env where there was never "ucx" present, so install_info.networks is not actually tightly controlled in every test, just due to oversight. I.e. "ucx" is explicitly added in tests intended to be "with ucx" but it's not explicitly removed in all other tests that just assume "without ucx" is the defatult state.

We should actually go through and remove the assumption that "no ucx" is always the "default".

See https://github.com/nv-legate/legate.core/actions/runs/6571421549/job/17850858678 for a case where this backfired, when ucx was added to the build.