Track Cabal change to its default installation directory (Windows)

[randen]

See haskell/cabal#4597. Resolving that referenced issue may result in the cabal executable being found in a different directory. If/when that happens, then we need to wait (maybe not: see following paragraphs) for the associated cabal.exe to be included in HP. The change would be to the installer template file (Nsfile.nsi.mu) for the Windows build of HP to reflect this.

There does seem to be some migration/compatibility concerns, so the Windows HP could work-around this somewhat by putting %LOCALAPPDATA%\cabal\bin on the user's %PATH% earlier than %AppData%\cabal\bin. Currently, we already put %AppData%\cabal\bin after everything else Haskell-related, in order to favor the just-installed HP files (or globally installed executables which are added by the user later).

So, if we do this %PATH% change to include both the %LOCALAPPDATA%\cabal\bin and %AppData%\cabal\bin (in the order per above), then I believe we could do it even before the Cabal change is made (assuming it will be done), as %LOCALAPPDATA%\cabal\bin is not assumed or used in the current Cabal, so it has no legacy and would, in theory, be harmless to include in the %PATH%.