Cirrus-CI: move QEMU pkg installation to test script

Occasionally the QEMU package fails to build and isn't available on
-CURRENT using the Latest package set. Move the package installation
to the test script, so that if the package isn't available we at least
still perform a build test.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 612f254c82468d037fccb6c26dc0a7e37a73a53e)