Cirrus-CI: build as an unprivileged user

The Cirrus-CI-provided working tree is owned by root. Leave that as is
for simplicity but build as an unprivileged user; this tests building
with an unmodifiable source tree as a side effect.

Continue running the smoke test as root for now, as it failed when run
as an unprivileged user - pkg reported "Fail to chmod
/usr/bin/.pkgtemp.lpq.dUHpEqPGJ9pq:Operation not permitted"

Sponsored by: The FreeBSD Foundation