19.03: fatal file collision with fish completions

[matklad]

Issue description

I am trying out the upcoming 19.03 channel, but sudo nixos-rebuild --switch fails for me with

building '/nix/store/796n1890hvgnm0rk5f1pg15zakiwhs3j-system-fish-completions.drv'...
collision between `/nix/store/c4mr5y71ri1i8ryv91q755ajkyrla96f-procps-3.3.15-fish-completions/uptime.fish' and `/nix/store/ybgxpd7yz9d6513gg3knwvlzfc0zy9a9-coreutils-8.30-fish-completions/uptime.fish'
builder for '/nix/store/796n1890hvgnm0rk5f1pg15zakiwhs3j-system-fish-completions.drv' failed with exit code 25
cannot build derivation '/nix/store/8nc4wd8ycm7j7mj18ifggx4jxzzbi54k-etc.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/njn8wnk2rqnnjx308z4zf4i2ag8dp54m-nixos-system-nixos-19.03.170684.07e2b59812d.drv': 1 dependencies couldn't be built
error: build of '/nix/store/njn8wnk2rqnnjx308z4zf4i2ag8dp54m-nixos-system-nixos-19.03.170684.07e2b59812d.drv' failed

Steps to reproduce

1. switch to 19.03 channel
2. have programs.fish.enable = true in configuration.nix
3. run nixos-rebuild --switch

Technical details

 - system: `"x86_64-linux"`
 - host os: `Linux 4.14.103, NixOS, 19.03.170684.07e2b59812d (Koi)`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.2`
 - channels(matklad): `""`
 - channels(root): `"nixos-19.03.170684.07e2b59812d"`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos`

[dotlambda]

The commit to blame was reverted in https://github.com/NixOS/nixpkgs/pull/56439.