cirrus.yml: serialise make check

We do this on our other platforms to make it easier to see what has
broken.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Li-Wen Hsu <lwhsu@FreeBSD.org>
Message-Id: <20200612190237.30436-19-alex.bennee@linaro.org>
1 file changed