docker: Re-add xz-utils

Previously this was installed as a dependency of mailcap, which was a
dependency of mime-support, which was a dependency of various things.
But apparently things depending on mime-support are transitioning to
media-types which doesn't have that dependency.

We need xz-utils for extracting `build.tar.xz` in CI, so make sure it's
installed.

tools/docker/Dockerfile

@@ -41,6 +41,7 @@ RUN --mount=type=cache,target=/var/lib/apt/lists/,sharing=private \
 		systemd-standalone-tmpfiles \
 		unzip \
 		vim \
+		xz-utils \
 		zip \
 	&& apt-get remove --purge --auto-remove -y gpg software-properties-common \
 	&& find /var/ -name '*-old' -delete && rm -rf /var/log/dpkg.log /var/log/alternatives.log /var/log/apt/ ~/.launchpadlib