[WIP] feature:- ghcup install --isolate DIR option

This merge request introduces the --isolate option that allows installing of tools to locations mentioned by users, disregarding the default ghcup dirs

differences i could gather:-

• it also doesn't need to make any symlinks that regular commands make
• doesn't care if a tool has a version already installed
• TBD ??

realted issue:- #157 (closed)