Fix git-latest ansible-lint errors

Reference: https://ansible-lint.readthedocs.io/en/latest/default_rules/#git-latest

Our variable naming is not necessarily consistent across roles.
I've tried to follow the naming conventions of each individual role.
All new variables are suffixed with `_version`, but the prefix may be
somewhat different.

roles/matrix-bridge-mx-puppet-groupme/defaults/main.yml

@@ -6,6 +6,7 @@ matrix_mx_puppet_groupme_enabled: true
 
 matrix_mx_puppet_groupme_container_image_self_build: false
 matrix_mx_puppet_groupme_container_image_self_build_repo: "https://gitlab.com/robintown/mx-puppet-groupme"
+matrix_mx_puppet_groupme_container_image_self_build_repo_version: "{{ 'main' if matrix_mx_puppet_groupme_version == 'latest' else matrix_mx_puppet_groupme_version }}"
 
 # Controls whether the mx-puppet-groupme container exposes its HTTP port (tcp/8437 in the container).
 #

roles/matrix-bridge-mx-puppet-groupme/tasks/setup_install.yml

@@ -81,6 +81,7 @@
 - name: Ensure MX Puppet Groupme repository is present on self build
   ansible.builtin.git:
     repo: "{{ matrix_mx_puppet_groupme_container_image_self_build_repo }}"
+    version: "{{ matrix_mx_puppet_groupme_container_image_self_build_repo_version }}"
     dest: "{{ matrix_mx_puppet_groupme_docker_src_files_path }}"
     force: "yes"
   become: true