Regression with ~/.local/share/nix/root used as a chroot store

[augu5te]

Describe the bug

When ~/.local/share/nix/root is used as a chroot store with nix-static, nix flake show failed

# nix flake show
path:/home/orichard/hello?lastModified=1711466600&narHash=sha256 IGkkheERaBI7UV65W3STBpbO1A79TJ4fTbzdlXRV5Zs%3D
error: path '/home/orichard/.local/share/nix/root/nix/store/fl6fyrl15ydxs1ga7k5scn4qfy72wd49-source' is not in the Nix store

Steps To Reproduce

1. Build nix-static to simplify test on machine without nix and nix store: nix build "github:NixOS/nix/598deb2b23bc59df61c92ea25745d675686f3991#packages.x86_64-linux.nix-static"
2. Copy result/bin/nix on machine without nix
3. Test with simple flake.nix

Regression
nix flake show works up to 2.20.5. With git bisect, the first bad commit: [598deb2]

  Date:   Mon Feb 26 15:08:08 2024 +0100                                                    
                                                                                          
    Use SourcePath for reading flake.{nix,lock}                                           
                                                                                          
    Flakes still reside in the Nix store (so there shouldn't be any change
    in behaviour), but they are now accessed via the rootFS              
    accessor. Since rootFS implements access checks, we no longer have to                 
    worry about flake.{nix,lock} or their parents being symlinks that                     
    escape from the flake.                                                                
                                                                                          
    Extracted from the lazy-trees branch.     

@edolstra, I suppose it's related to #6530. Contrary to #6853, this issue is blocking.

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