[Windows][Installer] Always use own xz

[Vladimir Lomov]

Hello, this is a bug report.

Yesterday I tried to help to install TeX Live on a notebook of a student
and faced with weird behavior of install-tl-windows.bat. The notebook
has a broken mingw (not sure msys or msys2) installation so xz.exe from
it doesn't work.

I thought that the installer on Windows always uses it's own wget.exe and
xz.exe but seems I was wrong. Are there any particular reasons to use
xz.exe and wget.exe found in PATH instead of own ones? If there are no
reasons then I think it is a bug and should be fixed, otherwise it might
give unexpected errors (I faced with that one).

P.S. The student shares the notebook with other person, so she doesn't
control it at full and doesn't know how to fix the broken mingw
installation.

---
WBR, Vladimir Lomov

-- 
"As an adolescent I aspired to lasting fame, I craved factual certainty,
and I thirsted for a meaningful vision of human life -- so I became a
scientist.  This is like becoming an archbishop so you can meet girls."
		-- Matt Cartmill
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