archive: exclude imported plan clones from artifact upload

When a plan imports plans from a remote repository, tmt clones that repository into work-*/import/plans/<plan>/, once per plan. RHEL tier testing imports plans this way, and each clone holds about 1,000 files, mostly .git objects and test sources. The largest Red Hat ranch requests reached about 657k files, nearly all of them these clones, which is what drives the NFS inode pressure on the artifacts share.

No results or logs live under import/. They stay in work-*/plans/, work-*/log.txt and the request root. Add --exclude=work-*/import/plans/ so the clones are no longer uploaded.

Assisted-by: Claude Code Signed-off-by: Miroslav Vadkerti mvadkert@redhat.com