remove niv residue

```fish
for d in {.,**/*}
    if [ ! -d $d ]; continue; end

    set from ./nix/sources.nix
    set to   ./npins

    set from (realpath -s $from --relative-to $d)
    set to   (realpath -s $to   --relative-to $d)

    echo "($d, $from, $to)"

    for f in $d/*
        if [ ! -f $f ]; continue; end
        sed -i "s=$from=$to=" $f
    end
end
```

nix/overlays/pin-isabelle-2023.nix

@@ -1,5 +1,5 @@
 let
-  sources = import ../sources.nix;
+  sources = import ../../npins;
 in
   _: _: {
     # Isabelle version 2023