software/gitlab: use shorter paths for gitaly sockets

When running resilience tests we have too deep paths:

    time="2024-01-24T01:32:14+01:00" level=fatal msg="load config" config_path=/srv/slapgrid/slappart22/t/eeq/i/0/tmp/inst/T2/srv/runner/instance/slappart0/etc/gitaly-config.toml error="socket could not be created in /srv/slapgrid/slappart22/t/eeq/i/0/tmp/inst/T2/srv/runner/instance/slappart0/var/gitaly/sockets/internal: listen unix /srv/slapgrid/slappart22/t/eeq/i/0/tmp/inst/T2/srv/runner/instance/slappart0/var/gitaly/sockets/internal/test-0c5c65ff.sock: bind: invalid argument"