Pass --create-home to `useradd salsaci`

This adds --create-home to the useradd call in salsa-ci.yml

Some tools expect the home directory to exist.

For example, git will error if the files it's working on are not owned by the current user.

It suggests running git config --global --add safe.directory /path/to/repo to override that.

So, I run it, but it says:

error: could not lock config file /home/salsaci/.gitconfig: No such file or directory