For update script parallelization, we have started calling builtins.toJSON
on updateScripts, which triggers evaluation of paths and therefore their copying
to Nix store. This breaks update scripts that assume that they exist in nixpkgs
like dwarf-fortress.

#61935

Let’s stringify the paths before JSONification to prevent the evaluation.

It makes little sense for update.nix to try to update overlays; for most people,
they will point to a read-only repository most of the cases.

Previously we did not handle non-existant paths making the program crash.
Let’s show a proper error.

Make calling update.py a little nicer

Previously, we escaped the old version in place for use in sed commands,
and then had to use that in error messages. We can do better.

No need to try evaluate the URL twice as introduced in the original fix
cce4868

grep was not actually properly included by common-updater-scripts before
0105058 but I am not sure why would we ever
need to add coreutils to PATH.

Make it clear that PWD is not fixed

maintainers/scripts/update.nix: various fixes