Cirrus-CI: retry pkg installation on failure

Pkg installation failed somewhat frequently, always at:

[62/104] Fetching jpeg-turbo-2.0.6.txz: .......... done
pkg: http://pkgmir.geo.freebsd.org/FreeBSD:13:amd64/quarterly/All/jbigkit-2.1_1.txz: No route to host

Move pkg installation to a script and retry once upon failure as a
(hopefully temporary) workaround.

Reviewed by: imp
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D30613

(cherry picked from commit dd41de95a84d979615a2ef11df6850622bf6184e)