gitlab-ci.yml: Compile some jobs out-of-tree

So far we only compiled all jobs in-tree in the gitlab-CI. For the code
that gets compiled twice (one time for 64-bit and one time for 32-bit
for example), we can easily move one of the two jobs to out-of-tree build
mode to increase the build test coverage a little bit.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20200731094139.9364-1-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>