texlive.combine: fix build

Recent merge of pull request #14681 broke
the build of some texlive components (e.g.: `xetex`)
that do not include the `./share/texmf/web2c/texmf.cnf`.
This fix should allow these component to build
as before by operating on this file only when
it exits.

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

@@ -66,6 +66,25 @@ in buildEnv {
     # TODO: perhaps do lua actions?
     # tried inspiration from install-tl, sub do_texmf_cnf
   ''
+    patchCnfLua() {
+      local cnfLua="$1"
+
+      if [ -e "$cnfLua" ]; then
+        local cnfLuaOrig="$(realpath "$cnfLua")"
+        rm ./texmfcnf.lua
+        sed \
+          -e 's,texmf-dist,texmf,g' \
+          -e 's,texmf-local,texmf,g' \
+          -e "s,\(TEXMFLOCAL[ ]*=[ ]*\)[^\,]*,\1\"$out/share/texmf\",g" \
+          -e "s,\$SELFAUTOLOC,$out,g" \
+          -e "s,selfautodir:/,$out/share/,g" \
+          -e "s,selfautodir:,$out/share/,g" \
+          -e "s,selfautoparent:/,$out/share/,g" \
+          -e "s,selfautoparent:,$out/share/,g" \
+          "$cnfLuaOrig" > "$cnfLua"
+      fi
+    }
+
     (
       cd ./share/texmf/web2c/
       local cnfOrig="$(realpath ./texmf.cnf)"
@@ -79,18 +98,7 @@ in buildEnv {
         -e "s,\$SELFAUTOGRANDPARENT,$out/share,g" \
         "$cnfOrig" > ./texmf.cnf
 
-      local cnfLuaOrig="$(realpath ./texmfcnf.lua)"
-      rm ./texmfcnf.lua
-      sed \
-        -e 's,texmf-dist,texmf,g' \
-        -e 's,texmf-local,texmf,g' \
-        -e "s,\(TEXMFLOCAL[ ]*=[ ]*\)[^\,]*,\1\"$out/share/texmf\",g" \
-        -e "s,\$SELFAUTOLOC,$out,g" \
-        -e "s,selfautodir:/,$out/share/,g" \
-        -e "s,selfautodir:,$out/share/,g" \
-        -e "s,selfautoparent:/,$out/share/,g" \
-        -e "s,selfautoparent:,$out/share/,g" \
-        "$cnfLuaOrig" > ./texmfcnf.lua
+      patchCnfLua "./texmfcnf.lua"
 
       rm updmap.cfg
     )