In preparation for removing $(DOCKER_SCRIPT) from the tests/tcg configuration
files, have Make use the same container engine that had been probed at
configure time.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20220929114231.583801-19-alex.bennee@linaro.org>