Install apt-cacher-ng from bullseye-backports (it's more recent)

This likely fixes the HTTP error 502 problem (#147)

roles/gitian/tasks/main.yml

@@ -13,7 +13,6 @@
 - name: Install Gitian dependencies.
   apt:
     name:
-      - apt-cacher-ng
       - bridge-utils
       - curl
       - debootstrap
@@ -30,6 +29,13 @@
     state: present
     update_cache: yes
 
+- name: Install more recent version of apt-cacher-ng
+  apt:
+    name: apt-cacher-ng
+    state: latest
+    update_cache: yes
+    default_release: bullseye-backports
+
 - name: Install ruamel.yaml
   pip:
     name: "ruamel.yaml>=0.15.0,<0.16.0"