gitlab_runner: Update concurreny math to reflect the new VM size

Fixes: 97bc3928 ("gitlab_runner: raise max memory to 2GB")

roles/gitlab_runner/templates/config.toml.j2

@@ -32,7 +32,7 @@ listen_address = ":9252"
   executor = "custom"
   builds_dir = "/builds"
   cache_dir = "/cache"
-  limit = {{ (ansible_memtotal_mb * 0.9 / 1024) | round | int }}
+  limit = {{ (ansible_memtotal_mb * 0.9 / 2048) | round | int }}
   environment = ["ARCHIVER_STAGING_DIR=/var/tmp"]
   [runners.custom]
     prepare_exec = "/usr/local/bin/libvirt-executor"