Allow user to disable default population of home directory for runner user

What does this MR do?

Introduce a new environment variable GITLAB_RUNNER_DISABLE_SKEL which will disalbe the population of skel when installing the runner through .deb/.fpm packages.

Takes into account all comments remaining from !1523 (closed)

Why was this MR needed?

In some cases, default dotfiles cause crashes of shell runner as installed with package repositories.
Debian 10 "Buster" notably has a .bash_logout which cause it. See forum post here and associated issues.

I'm also waiting for this fix, even if I would prefer without a envvar and that the default would be not creating the skeleton files. As I understand it, this "breaking change" will be for the 13.0 and not a 12.x

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Added tests for this feature/bug
• In case of conflicts with master - branch was rebased

What are the relevant issue numbers?

• Previous merge request !1523 (closed)
• Issues #1379 (closed)

/cc @steveazz @marcel.amirault