Cirrus-CI: Add descriptive task name

Previously it appeared only as "main" in places like GitHub's list
of checks run as part of a pull request.

MFC after: 1 week
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 903526542ac2309c08c769e517ea173a9f67cdb2)

"boot smoke test" dropped from task name for the MFC, because Cirrus
runs a build test only on stable/12 (see 05b9673637fe for details).