meta/pkg-build: fix step order
raco pkg catalog-archive: handle interrupted update

While `catalog-archive` cannot handle arbitrary corruption of
a previous archive run, it should be able to handle a state
due to interruption or other transient errors.

Merge to v6.1.
meta/pkg-build: avoid losing docs due to transient errors

When assembling documentation, preserve any documentation previously built
for a package whose build currently fails.