fix: clickhouse image building in the UK

CI was failing during `apt update` with the following error:

	E: The repository 'https://repo.clickhouse.com/deb/stable main/ Release' does not have a Release file.

Note that this error was occurring in the UK and Germany, but not in
France.

tutorcairn/templates/cairn/build/cairn-clickhouse/Dockerfile

@@ -1,6 +1,11 @@
 # https://hub.docker.com/r/yandex/clickhouse-server/tags
 FROM docker.io/yandex/clickhouse-server:22.1.3.7
 
+# The clickhouse repo is currently unavailable in some parts of the world. If we don't
+# remove this repo here then `apt update` will fail. Check if the problem is resolved with:
+# curl https://repo.yandex.ru/clickhouse/deb/stable/
+# The above command should be a 200, and not a 404.
+RUN rm /etc/apt/sources.list.d/clickhouse.list
 RUN apt update && apt install -y python3
 COPY ./scripts /scripts
 RUN chmod a+x /scripts/*