ci: Use debian testing for deb-src

Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Signed-off-by: Robert Foss <robert.foss@collabora.com>
Signed-off-by: Jakob Bornecrantz <jakob@collabora.com>

ci/Dockerfile

@@ -16,7 +16,7 @@ RUN echo '#!/bin/sh' > /usr/sbin/policy-rc.d
 RUN echo 'exit 101' >> /usr/sbin/policy-rc.d
 RUN chmod +x /usr/sbin/policy-rc.d
 
-RUN echo deb-src http://deb.debian.org/debian unstable main >> /etc/apt/sources.list
+RUN echo deb-src http://deb.debian.org/debian testing main >> /etc/apt/sources.list
 RUN apt-get update && \
     apt-get -y install ca-certificates && \
     apt-get -y install --no-install-recommends \