gitlab_runner: Move VM memory to a variable instead of hardcoding it twice

It was forgotten once[1] to update it in both places, so avoid that
issue in the future, by moving it to a variable.

[1] c370c9d0 ("gitlab_runner: Update concurreny math to reflect the new VM size")

roles/gitlab_runner/defaults/main.yml

+gitlab_runner_libvirt_vm_memory: 2048

roles/gitlab_runner/tasks/main.yml

@@ -79,10 +79,12 @@
       copy: src={{ item.src }} dest={{ item.dest }} owner=root group=root mode={{ item.mode }}
       loop:
         - {src: arch-boxes.asc, dest: /usr/local/lib/libvirt-executor/, mode: 644}
-        - {src: domain_template.xml, dest: /usr/local/lib/libvirt-executor/, mode: 644}
         - {src: libvirt-executor, dest: /usr/local/bin/, mode: 755}
         - {src: libvirt-executor-update-base-image, dest: /usr/local/bin/, mode: 755}
 
+    - name: Install libvirt-executor domain template
+      template: src=domain_template.xml.j2 dest=/usr/local/lib/libvirt-executor/domain_template.xml owner=root group=root mode=0644
+
     - name: Create SSH keys for libvirt-executor
       command: ssh-keygen -N "" -f /etc/libvirt-executor/id_ed25519 -t ed25519
       args:

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 / 2048) | round | int }}
+  limit = {{ (ansible_memtotal_mb * 0.9 / gitlab_runner_libvirt_vm_memory) | round | int }}
   environment = ["ARCHIVER_STAGING_DIR=/var/tmp"]
   [runners.custom]
     prepare_exec = "/usr/local/bin/libvirt-executor"

roles/gitlab_runner/files/domain_template.xml → roles/gitlab_runner/templates/domain_template.xml.j2

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