Prevent recursion in environment variable expansion

Ensure that we don't get stuck in an endless recursion while
expanding variables. Handle both the simple case of `VAR=$VAR`
but also more complex scenarios such as `VAR1=$VAR2; VAR2=$VAR2`.

Based on the patch by David Redondo and input from Igor Kushnir,
see https://invent.kde.org/kde/kdevelop/-/merge_requests/125