Fix self-building for Postmoogle

Before this, it was trying to pull the `latest` git branch.

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/2043

roles/matrix-bot-postmoogle/defaults/main.yml

@@ -6,7 +6,7 @@ matrix_bot_postmoogle_enabled: true
 
 matrix_bot_postmoogle_container_image_self_build: false
 matrix_bot_postmoogle_docker_repo: "https://gitlab.com/etke.cc/postmoogle.git"
-matrix_bot_postmoogle_docker_repo_version: "{{ matrix_bot_postmoogle_version }}"
+matrix_bot_postmoogle_docker_repo_version: "{{ 'main' if matrix_bot_postmoogle_version == 'latest' else matrix_bot_postmoogle_version }}"
 matrix_bot_postmoogle_docker_src_files_path: "{{ matrix_base_data_path }}/postmoogle/docker-src"
 
 matrix_bot_postmoogle_version: latest