tugger-wix: Make msi option to add to path env optional.

[andrewleech]

When building / installing a GUI application, it's less common to want the install folder added to system env PATH.
This PR makes the inclusion of this checkbox in the msi installer optional, though enabled by default to not effect existing users.

Used like:

def make_msi(exe):
    msi = exe.to_wix_msi_builder(
        "wsl_usb_gui",
        "WSL USB",
        VARS.get("version"),
        "Andrew Leech"
    )
    msi.add_to_path = False