Merge #55420: buildEnv, substituteAll: disable binary cache

... into staging.

pkgs/build-support/buildenv/default.nix

@@ -70,6 +70,7 @@ runCommand name
       priority = drv.meta.priority or 5;
     }) paths);
     preferLocalBuild = true;
+    allowSubstitutes = false;
     # XXX: The size is somewhat arbitrary
     passAsFile = if builtins.stringLength pkgs >= 128*1024 then [ "pkgs" ] else null;
   }