Motivation for this change

Many of the builtin scripts provided with i3blocks depend on packages not available by default.
This change makes sure that the various scripts can access their own dependencies.

Additionally, an parameter scripts has been added to select which scripts to provide the
dependencies of. By default, the dependencies of all defaults will be installed.
To change this behavior override scripts with a list of string corresponding to the desired packages.

TODO:

• Provide better interface for selecting scripts.
• Provide better default. (currently: all)
• Don't install undesired scripts.
• Select FreeBSD tools when on FreeBSD

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
  • Linux
• 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.