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/.

pkgs/tools/typesetting/tex/texlive/combine.nix

@@ -153,7 +153,8 @@ in buildEnv {
       rm "$link"
       makeWrapper "$target" "$link" \
         --prefix PATH : "$out/bin:${perl}/bin" \
-        --prefix PERL5LIB : "$PERL5LIB"
+        --prefix PERL5LIB : "$PERL5LIB" \
+        --set-default TEXMFCNF "$TEXMFCNF"
 
       # avoid using non-nix shebang in $target by calling interpreter
       if [[ "$(head -c 2 "$target")" = "#!" ]]; then