Resolve the build_root before we append more items onto it so that the
environment output is more concise with less parent directory confetti
in it.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20230621002121.1609612-4-jsnow@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>