ci: Update reason why arm64 crossbuild is disabled

ci: Update reason why arm64 crossbuild is disabled

The old reason why it was disabled was bug 982406 'mark markdown
Multi-Arch: foreign', but that was recently fixed.

Trying to enable it revealed another reason why it still doesn't work:
$ eatmydata apt-get build-dep ${HOST_ARCH:+--host-architecture ${HOST_ARCH} -Pcross,nocheck} --no-install-recommends -y $aptopts .
...
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
 ocaml:arm64 : Depends: gcc:arm64 but it is not installable
               Depends: binutils:arm64 but it is not installable
E: Unable to correct problems, you have held broken packages.