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