bindist: Use complete relative paths when cding to directories
If a user has configured CDPATH on their system then cd lib
may change
into an unexpected directory during the installation process.
If you write cd ./lib
then it will not consult CDPATH
to determine
what you mean.
I have added a check on ghcup-ci to verify that the bindist installation works in this situation.
Fixes #24951 (closed)