Motivation for this change

Currently singularity-tools will try to copy the bin contents of the entire transitive closure of packages requested for the singularity container. This can cause unimportant collisions between binaries that the user didn't intend to be installed in the PATH anyway. This patch makes it so that while the entire /nix/store is copied into the container, only the requested contents are exposed in /bin.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• 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 nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.