Use on non-Nix base OS

[alerque]

I use Nix quite a bit for projects and remote CI, but I run Arch Linux as a base system on my bare metal.

I do have flakes enabled (and working for many other projects) and also added the extra substituters and public keys to my trusted user's nix.conf.

Running nix run github:ryantm/nixpkgs-update -- --help seems to work, but I can't get any actual update action going on. Interestingly I get different results if I run it in the root of a Git working tree of the nixpkgs repository vs just somewhere else. Here is the somewhere else, which downloads a copy to ~/.cache/nixpkgs:

$ nix run github:ryantm/nixpkgs-update -- update --pr "sile 0.14.12 0.14.13 repology.org"
[options] github_user: alerque, pull_request: YES, batch_update: NO, calculate_outpaths: NO, cve_report: NO, nixpkgs-review: NO, nixpkgs_dir: "/home/caleb/.cache/nixpkgs", use attrpath: NO
For attrpath sile, using log file: /run/user/1000/nixpkgs-update/sile/2023-11-01.log
sile 0.14.12 -> 0.14.13 repology.org
attrpath: sile
nixpkgs-update: pkgs/tools/typesetting/sile/default.nix: hGetContents: invalid argument (invalid byte sequence)

Here is the alternative if I run from my up to date working tree (master branch):

$ nix run github:ryantm/nixpkgs-update -- update --pr "sile 0.14.12 0.14.13 repology.org"
[options] github_user: alerque, pull_request: YES, batch_update: NO, calculate_outpaths: NO, cve_report: NO, nixpkgs-review: NO, nixpkgs_dir: "/home/caleb/projects/distro-packaging/nix", use attrpath: NO
For attrpath sile, using log file: /run/user/1000/nixpkgs-update/sile/2023-11-01.log
sile 0.14.12 -> 0.14.13 repology.org
attrpath: sile
Old version 0.14.12" not present in staging derivation file with contents:
...

What follows that is a dump of the default.nix file for sile, but very strangely it is about 6 versions old. It is not the version in the current repository, it is 0.14.5.

What am I doing wrong here? Or is this a function that this tool makes assumptions about the host environment and must be run under NixOS?