Windows: Use %LOCALAPPDATA%/zigup as the default install directory.

[jhark]

The previous behaviour was to use the zigup executable's own directory.

The previous behaviour would result in both a zig directory, and a zig.exe file in the user's PATH. The directory would prevent the linked zig executable created by zigup from being found in PATH when running under MSYS (e.g. trying to use zigup from within the bash install that comes with git for windows). Using %LOCALAPPDATA% avoids this issue and better matches the behaviour on other platforms.

Tested on Windows and Linux (under WSL) with LOCALAPPDATA/HOME:

• Unset.
• Set to a bad path.
• Set normally.

Observed expected error output/success.