scala-runners: init at unstable-2020-02-02

[hrhino]

Resubmission of #84068 (apparently you can't push changes and then reopen a PR on github?)

Motivation for this change

It's a good script and useful for testing with pre-release and non-current versions of Scala.

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/)
• 21.11 Release Notes (or backporting 21.05 Relase notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

[Artturin]

the title of the pr and commit should be scala-runners: init at unstable-2020-02-02

[hrhino]

As it now is. Thanks.