.cirrus.yml: Refactor

Most of the matrix options are manually triggered ones but some of those
do not specifically mention them in the name. So add the string
' (manual)` in the name of the jobs for better clarity in the web
interface of cirrus CI.

These are mostly cosmetic changes and have no effects on the build
itself.

Will be merged in all stable versions.

Approved by: emaste
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50110

(cherry picked from commit 5c67180ffa931242a98b1a747e811f2acbd351db)