top-level: Make config-overriden stdenv bootstrap more normally

pkgs/stdenv/custom/default.nix

@@ -0,0 +1,17 @@
+{ system, allPackages, platform, crossSystem, config, ... } @ args:
+
+rec {
+  vanillaStdenv = (import ../. (args // {
+    # Remove config.replaceStdenv to ensure termination.
+    config = builtins.removeAttrs config [ "replaceStdenv" ];
+  })).stdenv;
+
+  buildPackages = allPackages {
+    # It's OK to change the built-time dependencies
+    allowCustomOverrides = true;
+    bootStdenv = vanillaStdenv;
+    inherit system platform crossSystem config;
+  };
+
+  stdenvCustom = config.replaceStdenv { pkgs = buildPackages; };
+}