Force self-building of matrix-hookshot on arm64

The `halfshot/matrix-hookshot` container images published to Docker Hub
(as of 2022-04-05, at least) are only available for `amd64`, not for
`arm64`. Self-building on arm64 is necessary.

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1728

It should be noted that a `roiarthurb/matrix-hookshot` container image is available,
which is available for the arm64 platform, but that's non-official and doesn't
contain an amd64 build, so it's of limited use.

group_vars/matrix_servers

@@ -671,7 +671,7 @@ matrix_heisenbridge_systemd_wanted_services_list: |
 # We don't enable bridges by default.
 matrix_hookshot_enabled: false
 
-matrix_hookshot_container_image_self_build: "{{ matrix_architecture not in ['arm64', 'amd64'] }}"
+matrix_hookshot_container_image_self_build: "{{ matrix_architecture not in ['amd64'] }}"
 
 matrix_hookshot_appservice_token: "{{ '%s' | format(matrix_homeserver_generic_secret_key) | password_hash('sha512', 'hookshot.as.tok') | to_uuid }}"