texlive.combine: set TEXMFCNF in binary wrapper
This helps kpathsea to find texmf.cnf in some cases. For example,
dvipng was trying to look for it in
/nix/store/<hash>-texlive-dvipng.bin-2019/ instead of
/nix/store/<hash>-texlive-combined-full-2019/.
(cherry picked from commit 91c9f2ab5c931ad036f6a5d1872c22d203b751af)
cc #83816