pkgs/stdenv: fix defaultUnpack on nfs

-p preserves permission,mode,timestamps.

When trying to copy with permission on NFS, it can fail;

The closest documentation I found on this was:

https://access.redhat.com/solutions/725513

One suggestion is to change the netapp configuration, but another that
should work for everyone is to use the cp command presented in this
patch.

With this, builds on nfs work for me. I however cannot say that I'm
certain that removing the permission preservation won't cause any other
troubles elsewhere, e.g. by perhaps introducing some sort of
non-determinism.

pkgs/stdenv/generic/setup.sh

@@ -1211,7 +1211,7 @@ _defaultUnpack() {
         # We can't preserve hardlinks because they may have been
         # introduced by store optimization, which might break things
         # in the build.
-        cp -pr --reflink=auto -- "$fn" "$destination"
+        cp -r --preserve=mode,timestamps --reflink=auto -- "$fn" "$destination"
 
     else