Remote build with flakes fails

[asymmetric]

Describe the bug
Running nixos-rebuild switch --target host foo [--build-host foo] .#foo errors:

❯ nixos-rebuild switch --target-host wireguard-gateway --flake ..#wireguard-gateway
warning: 'nix flake info' is a deprecated alias for 'nix flake metadata'
warning: Git tree '/home/asymmetric/code/centrifuge/deployment' is dirty
building the system configuration...
warning: Git tree '/home/asymmetric/code/centrifuge/deployment' is dirty
don't know how to build these paths:
  /nix/store/p1czwcvnh3zljs1ymwqh3gvab2djapkk-nixos-system-unnamed-21.05.20210412.311ceed
error: path '/nix/store/p1czwcvnh3zljs1ymwqh3gvab2djapkk-nixos-system-unnamed-21.05.20210412.311ceed' does not exist and cannot be created

To Reproduce
Steps to reproduce the behavior:

1. have a flake with a nixosConfigurations
2. run nixos-rebuild switch --target-host foo --flake .#foo

Expected behavior
The closure is copied to the remote host, and the generation is activated.

Screenshots
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Additional context
There's also a conversation here.

Notify maintainers

Metadata

❯ nix-shell -p nix-info --run "nix-info -m"
this path will be fetched (0.05 MiB download, 0.28 MiB unpacked):
  /nix/store/qgbwdnk91rk26b5bkd6qv5r6c2v733kb-bash-interactive-4.4-p23-dev
copying path '/nix/store/qgbwdnk91rk26b5bkd6qv5r6c2v733kb-bash-interactive-4.4-p23-dev' from 'https://cache.nixos.org'...
 - system: `"x86_64-linux"`
 - host os: `Linux 5.4.105, NixOS, 20.09.20210412.c3253cd (Nightingale)`
 - multi-user?: `yes`
 - sandbox: `no`
 - version: `nix-env (Nix) 2.4pre20210326_dd77f71`
 - channels(root): `"nixos-20.09.3346.4d0ee90c6e2"`
 - channels(asymmetric): `""`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos`

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute:
# a list of nixos modules affected by the problem
module:

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/building-a-flake-based-nixos-system-remotely/11309/4

[rnhmjoj]

I believe this has been fixed with #119540.