CI: wiki: adjust for 'pmbootstrap ci' (MR 3615)

Add a wrapper .ci/wiki.sh, that pmbootstrap ci can run and use it in
gitlab-ci.yml. Add a .ci/lib/gitlab_prepare_ci.sh script that makes a
user available with sudo, as expected by the pmbootstrap ci compatible
scripts. Set up binfmt_misc too while at it, it will be needed for
running pmbootstrap.

The next patches will adjust the rest of the CI scripts and move the old
scripts in the .ci/lib dir, so only the scripts that should be called
directly are in the .ci dir.

Related: https://postmarketos.org/pmb-ci