Update tor upgrade check for newer aptitude

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. Let's ignore the exit code, since what we care
about is whether "tor" is in the stdout.

We could use `apt list --upgradable`, but that command warns about
lack of a stable CLI, and additionally we'd have to munge the output
further to inspect it.

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

@@ -20,6 +20,8 @@
   command: aptitude search '~U' --display-format '%p' --disable-columns
   register: tor_upgradable_result
   changed_when: false
+  # aptitude >= 0.7.6 will exit non-zero if no hits
+  failed_when: false
 
 - name: Hold tor package to prevent upgrade breaking SSH connection.
   command: aptitude hold tor