nix-portable breaks on symlinked home directory

[erooke]

version: nix-portable-010

I am trying to setup nix-portable on an HPC cluster and it turns out that our
configured home directories are links somewhere else. This breaks nix-portable
when it tries to call bwrap:

$ ./nix-portable nix run nixpkgs#hello

bwrap: Can't mkdir parents for /Users/erooke/.nix-portable/ca-bundle.crt: No such file or directory

$ HOME=$(readlink -f $HOME) ./nix-portable nix run nixpkgs#hello

Hello, world!

I don't know how common of a setup this is or if there is interest in fixing it here but it took me a hot minute to figure out what was going wrong.