gitlab_runner: raise max memory to 2GB

This is required f.e. for the dbscripts pipeline where kcov coverage
requires quite a bit more memory and fails to run in parallel with 1GB
limit.

roles/gitlab_runner/files/domain_template.xml

 <domain type='kvm'>
   <name>$vm_name</name>
-  <memory unit='MiB'>1024</memory>
+  <memory unit='MiB'>2048</memory>
   <vcpu>4</vcpu>
   <os>
     <type arch='x86_64' machine='q35'>hvm</type>