Describe the bug

This is duplicating NixOS/nixpkgs#132732 since I think it is a nix issue.
The issue was introduced with my Pull Request NixOS/nixpkgs#131102

Essentially, when environment.etc.?.source refers to a path (./zinputrc in this case), it seems like the path the link is referring to is either not existant at all or it gets garbage collected.

Steps To Reproduce

These are alread described in NixOS/nixpkgs#132732

Expected behavior

The symlink should be valid.

nix-env --version output

Not working: nix-env (Nix) 2.4pre20210707_02dd6bb

Working: nix-env (Nix) 2.3.15

Additional context

Add any other context about the problem here.
cc @Mic92