Changes

Salsa-CI: Extend autopkgtest job timeout to 3h by default

Looking at current open MRs for Lintian, nearly all of them fail on
autopkgtest timing out. Seems contributors are not looking at the
CI output and reading the error. Fix this by forcing the timeout to 3h
directly from within the GitLab CI definition.

This renders the existing `before_script` section obsolete, but keep it
still around as fall-back to satisfy those who have concerns that the
`timeout` might not always work.