Follow symlinks in the Win32 code for System.Environment.getExecutablePath

This partially addresses #14483 by fixing the Windows implementation of
System.Environment.getExecutablePath. This is achieved by using
GetFinalPathNameByHandleW to resolve potential symlinks, while making
sure we do not get back a UNC path (see #14460).

Test Plan: Validate

Reviewers: Phyx, bgamari, angerman, hvr, goldfire

Reviewed By: Phyx, bgamari

GHC Trac Issues: #14483

Differential Revision: https://phabricator.haskell.org/D4227