flake.nix: Only add `_file`-key if position of `args.modules` is actually known to the evaluator
flake.nix: Only add `_file`-key if position of `args.modules` is actually known to the evaluator

This happens if the evaluator "loses" the position of an
attr-declaration[1] because of e.g. too many nested function-calls to
build the final attr-set.

While the actual issue should be fixed in Nix itself, this is IMHO a
fair workaround to unblock affected users[2].

[1] https://github.com/NixOS/nixpkgs/commit/e14c24593420bb9057e7f38b40d17137eaeff9dd#commitcomment-53645936
[2] It seems as everyone using `divnix/digga` or `flake-utils-plus`
    are affected:
    * https://github.com/divnix/digga/issues/87

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