gitlab-ci: workaround build failure in Debian:sid due to iproute2 issue

Our platform unit tests try to add an IP tunnel using iproute2.
That fails with

    "add tunnel "ip6tnl0" failed: File exists"

This is a bug in iproute2-5.2.0, see [1].

Workaround the issue by downgrading the package.

[1] https://www.spinics.net/lists/netdev/msg584916.html

.gitlab-ci.yml

@@ -43,6 +43,10 @@ stages:
     - date '+%Y%m%d-%H%M%S'; pip3 install meson
     - date '+%Y%m%d-%H%M%S'; contrib/scripts/nm-ci-install-valgrind-in-ubuntu1604.sh
 
+    # iproute2 5.2.0 on debian:sid causes our unit tests to fail.
+    # Downgrade to a working version. See https://www.spinics.net/lists/netdev/msg584916.html
+    - date '+%Y%m%d-%H%M%S'; ! ( dpkg -s iproute2 | grep -q '^Version[:] 5.2.0-1$' ) || (curl 'http://ftp.debian.org/debian/pool/main/i/iproute2/iproute2_4.20.0-2_amd64.deb' --output /tmp/iproute2_4.20.0-2_amd64.deb && dpkg -i /tmp/iproute2_4.20.0-2_amd64.deb)
+
 .do_build: &do_build
   stage: test
   script: