maintainers/scripts/update.nix: Run update scripts from the worktree

`update.nix` extracts `passthru.updateScript` attributes in the main repo
and when they are relative paths (e.g. `./update.sh`), Nix will resolve them
to absolute paths in the main repo.

Update scripts can use $(dirname $0) to get the location of files they
should update but that would point to the main repo.
We want them to modify the appropriate git worktree instead
so we replace the prefix accordingly.

`git rev-parse --show-toplevel` will resolve symlinks but, fortunately,
Nix will do that as well, so the path will match:

https://github.com/NixOS/nixpkgs/pull/98304#issuecomment-695761754

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