• The -overlay flag runs the specified binary inside an OverlayFS,
  since the /nix store may be in a different mount point than the user
  home, this patch explicitly bind mounts it so it's available inside
  the overlay.

• profile builder: firejail provides facilities to build a new profiles.
  To do so, it execute the helper binary fbuilder, which in turn will
  execute firejail back with different options. This patch makes it use
  the binary available in PATH instead of the one produced at compile time.
  The compiled firejail binary doesn't have the necessary permissions,
  so the firejail NixOS module wraps it in a SUID wrapper available on
  PATH at runtime.

Signed-off-by: Roosembert Palacios roosemberth@posteo.ch

Mind that testing with nixos-rebuild build-vm does not work because build-vm itself uses an overlayfs to mount the nix store.

Motivation for this change

Fixing non-working functionality of firejail.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.