AttrCursor::getStringWithContext(): Force re-evaluation if the cached context is not valid
AttrCursor::getStringWithContext(): Force re-evaluation if the cached context is not valid

Fixes #4236.

NixOS/nix,rycee/nix,rycee/nix,NixOS/nix,NixOS/nix,rycee/nix,rycee/nix,NixOS/nix,rycee/nix,NixOS/nix