Fix CURRENT_RELEASE URL; rebuild metpo-base.* with correct terms

Adds CURRENT_RELEASE override to metpo.Makefile so release_diff
downloads https://w3id.org/metpo/metpo.owl (correct) rather than
the generated https://w3id.org/metpo.owl (404).

Rebuilds metpo-base.* after the base-artifact filter fix (#463):
these files now contain METPO terms instead of being empty.
The main and full artifacts are unchanged from the prior commit.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: turbomam