CI: fix BIRD cert issues on older Debian systems
On Debian 9:
Cloning into 'bird'... fatal: unable to access 'http://gitlab.nic.cz/labs/bird.git/': server certificate verification failed. CAfile: /etc/ssl/certs/ca-certificates.crt CRLfile: none
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On Debian 9:
Cloning into 'bird'... fatal: unable to access 'http://gitlab.nic.cz/labs/bird.git/': server certificate verification failed. CAfile: /etc/ssl/certs/ca-certificates.crt CRLfile: none