texlivePackages.l3build: inject path to system texmf.cnf

pkgs/tools/typesetting/tex/texlive/fixed-hashes.nix

@@ -9749,7 +9749,7 @@
     source = "12np4gmh0y5cg66ijb96kfbdq49d38rg485nql5q0n6pgn42apnc";
   };
   l3build-77170 = {
-    run = "0f3qfgm4whm1p8alx8gwca2glgx86c48w04d62zkpzgpadkyhzbq";
+    run = "18104w97ad97ki02yghqclcdv0v916h8lyp676cyg3wc7hjvs5cq";
     doc = "0i45c0bni3xx38wgcp30gh2dg0ak2snsfjrx86kwfyvmgjsm9ric";
     source = "1n2g8vcfnya74dsm8plaj0vbfwgs8a4lqpxqvxa9vgdda288534p";
   };

pkgs/tools/typesetting/tex/texlive/tlpdb-overrides.nix

@@ -460,6 +460,15 @@ lib.recursiveUpdate orig rec {
     substituteInPlace "$out"/bin/latexindent --replace-fail 'use FindBin;' "BEGIN { \$0 = '$scriptsFolder' . '/latexindent.pl'; }; use FindBin;"
   '';
 
+  # l3build ignores the TEXMFCNF variable to prevent user customisations from affecting the build
+  # but we rely on TEXMFCNF to find the system texmf.cnf, so we must inject its path into l3build
+  # WARNING: this relies on the system texmf.cnf being in $TEXMFSYSVAR/web2c
+  l3build.postUnpack = ''
+    if [[ -f "$out"/scripts/l3build/l3build-aux.lua ]] ; then
+      substituteInPlace "$out"/scripts/l3build/l3build-aux.lua --replace-fail '" TEXMFCNF=."' '" TEXMFCNF=." .. os_pathsep .. kpse.var_value("TEXMFSYSVAR") .. "/web2c"'
+    fi
+  '';
+
   # find files in script directory, not in binary directory
   minted.postFixup = ''
     substituteInPlace "$out"/bin/latexminted --replace-fail "__file__" "\"$scriptsFolder/latexminted.py\""