Hmm, we've actually hardcoded fusermount in kbfs

• nixpkgs/pkgs/tools/security/keybase/fix-paths-kbfs.patch

  Line 10 in 9995881

  + fusermountOutput, fusermountErr := exec.Command("@fusermount@", "-u", dir).CombinedOutput()

But I think this might have had unintended side-effects.
I believe this needs to be suid, which it won't when it's in the /nix/store.
That's why the path here is at /run/wrappers.

Also, it's perfectly fine to assume this in NixOS because the following configuration will be applied on every system using NixOS

• nixpkgs/nixos/modules/security/wrappers/default.nix

  Line 159 in c81e4f1

  fusermount.source = "${pkgs.fuse}/bin/fusermount";

@worldofpeace thanks for the response and explanation.

It seems something is broken in my current setup. I'll close this PR for now while I investigate it.