ci: build out-of-tree

ci: build out-of-tree

Most developers are using out-of-tree builds and it was discussed in the past
to only allow those.  To prepare for the transition, use out-of-tree builds
in all continuous integration jobs.

Based on a patch by Marc-André Lureau.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Li-Wen Hsu <lwhsu@freebsd.org>
Message-Id: <1576074829-56711-1-git-send-email-pbonzini@redhat.com>