launching buildFHSUserEnvBubblewrap fails when / contains directory with spaces

[bennofs]

Describe the bug
When you have directories with spaces in your root directory /, then the launch script of buildFHSUserEnvBubblewrap fails since it does not correctly escape the space.

To Reproduce

$ sudo mkdir '/with space'
$ nix run '(with import <nixpkgs> {}; buildFHSUserEnvBubblewrap { name = "example"; })' -c 'example'
bwrap: Can't find source path /with: No such file or directory
$ 

Expected behavior
The launcher script should work correctly even with directory names that contain spaces.

Additional context
Discovered when trying to run steam, which also uses buildFHSUserEnvBubblewrap.

Notify maintainers
@illegalprime

Metadata

• system: "x86_64-linux"
• host os: Linux 5.9.1-arch1-1, Arch Linux, noversion
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.3.4
• channels(bennofs): ""
• channels(root): "nixpkgs-21.03pre248787.7ad5e816fab"
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixpkgs