Switch tor upgrade check from aptitude to apt

Between the version of aptitude on Xenial and that on Focal, empty
searches started returning 1, causing `Check whether tor will be
upgraded` to fail. Replacing the aptitude search with `apt list
--upgradable tor` should work on both releases, and is more specific.

install_files/ansible-base/roles/common/tasks/apt_upgrade.yml

@@ -17,7 +17,7 @@
   tags: apt
 
 - name: Check whether tor will be upgraded.
-  command: aptitude search '~U' --display-format '%p' --disable-columns
+  command: apt list --upgradable tor
   register: tor_upgradable_result
   changed_when: false