ci: upgrade conformance to regression test

[buffalojoec]

The "conformance" tests were already just using Mollusk's CLI to ensure no regressions across program fixtures between the cached previous version of the program and the current program source.

This PR renames the test to "regression" to more accurately reflect its purpose, then makes a few updates to more comfortably use the Mollusk CLI tooling:

• Use Mollusk fixtures instead of Firedancer fixtures (regenerated). No sense using FD fixtures if we aren't using FD tooling anymore.
• Adjust the CLI command to ignore CUs, since we have been updating dependencies.