@waffle8946 is referring to a commit from 5 hours ago. You should never touch that value (like the comment right above it says), see this post for details see the wiki entry I just updated for details: FAQ/When do I update stateVersion - NixOS Wiki
The change that ended up being made to it is at least that now you must specify a valid NixOS version, but I still think forcing it to be a hash would prevent mistakes like this… Anyway, this is besides the problem you’re having right now.
Hm, if you’re running that the contents of $NIX_PATH don’t actually matter, but a simple echo $NIX_PATH would do 
You’re sure that directory contains the exact same repository as the commit you’ve just pushed? Again, there is no reference to that file:
~/Documents/Projects/fifis_flakes> rg hardware-configuration
flake.nix
20: ./thinkbug/hardware-configuration.nix
36: ./oystertop/hardware-configuration.nix
And actually evaluating your configuration works just fine:
~/Documents/Projects/fifis_flakes> nixos-rebuild dry-build --flake .#thinkbug
building the system configuration...
warning: Git tree '/home/tlater/Documents/Projects/fifis_flakes' is dirty
these 385 derivations will be built:
/nix/store/00ggwi7kcxh7q83jlrma9p53b4rhc79a-check-link-targets.sh.drv
/nix/store/m0k29s59q5k2wawa3s4aj0d37hcb4d91-builder.pl.drv
...
I.e., if you were using the repository on codeberg, your configuration should evaluate just fine. You have either indeed stumbled upon a rather massive nix bug, or you’re not actually running the code you’ve published.
Actually, you could just do this:
$ nixos-rebuild switch \
--flake 'git+https://codeberg.org/filemon_filemon/fifis_flakes#thinkbug' \
--use-remote-sudo