Nix gets broken if a substituter is unreachable

[Rida-123456]

Describe the bug

I cannot run any nix command:

$ nix run nixpkgs#hello
error: failed to start SSH connection to 'eu.nixbuild.net'

[NaN-git]

This seems to be a duplicate of #12681

[Mic92]

Agreed same as the other issue.