Motivation for this change

Before this change, Idris would silently build all files of a package without indicating any progress until everything is done, because it doesn't seem to flush on newlines when the output isn't a tty. This is especially frustrating because Idris can take a very long time to compile things.

Ping @brainrape @mpickering @shlevy

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option 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/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Fits CONTRIBUTING.md.