Do not use matrix:matrix for Dimension configuration

Dimension runs as the `node` user in the container (`1000:1000`).
It doesn't seem like we have a way around it. Thus, its configuration
must also be readable by that user (or group, in this case).

roles/matrix-dimension/tasks/setup_dimension.yml

@@ -10,7 +10,7 @@
     state: directory
     mode: 0770
     owner: "{{ matrix_user_username }}"
-    group: "{{ matrix_user_groupname }}"
+    group: "{{ matrix_dimension_user_gid }}"
   when: matrix_dimension_enabled|bool
 
 - name: Ensure Dimension config installed
@@ -19,7 +19,7 @@
     dest: "{{ matrix_dimension_base_path }}/config.yaml"
     mode: 0640
     owner: "{{ matrix_user_username }}"
-    group: "{{ matrix_user_groupname }}"
+    group: "{{ matrix_dimension_user_gid }}"
   when: matrix_dimension_enabled|bool
 
 - name: Ensure Dimension image is pulled