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

(cherry picked from commit 383c01a8)