nix-portable does not fallback to proot when user namespaces are not available

[algal]

The readme file says:

If user namespaces are not available on a system, nix-portable will fall back to using proot as an alternative mechanism to virtualize /nix.

But when I follow the install instructions exactly and try to use it on a Ubuntu 22.04.05 machine, I get this:

(base) ag@tiny2:~$ ./nix-portable nix run nixpkgs#htop
error: setting up a private mount namespace: Operation not permitted

So it seems not to fallback correctly.