libxml2: build with the bootstrap stdenv on Darwin

Using the bootstrap stdenv avoids an infinite recursion from xcbuild
depending on libxml2 depending on xcrun from xcbuild, which is
propagated by the non-bootstrap stdenv.

pkgs/top-level/all-packages.nix

@@ -21800,6 +21800,13 @@ with pkgs;
 
   libxml2 = callPackage ../development/libraries/libxml2 {
     python = python3;
+    stdenv =
+      # libxml2 is a dependency of xcbuild. Avoid an infinite recursion by using a bootstrap stdenv
+      # that does not propagate xcrun.
+      if stdenv.hostPlatform.isDarwin then
+        darwin.bootstrapStdenv
+      else
+        stdenv;
   };
 
   libxml2Python = let