maintainers/scripts/update.nix: Fix deduplication for unstableGitUpdater
maintainers/scripts/update.nix: Fix deduplication for unstableGitUpdater

Some updaters like `unstableGitUpdater` rely solely `UPDATE_NIX_ATTR_PATH`
environment variable to find out package attribute so they all have
the same `passthru.updateScript`. This means we cannot rely
on that attribute to filter out aliased packages because it would
consider all packages using `unstableGitUpdater` the same.

We need to use different criterion to distinguish them. `meta.position`
is a reasonable candidate but we cannot always utilize it since
it might not be available or it might be shared among multiple packages
(e.g. when using custom mkDerivation wrapper).

Combination of the two attributes should distinguish most cases.

Fixes: https://github.com/NixOS/nixpkgs/issues/112583

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