iosevka: don't show build progress

[cole-h]

Motivation for this change

The node progress "bar" looks like crap when using nix-build. Redirect
the infinite scrolling screen to /dev/null to calm the output down.
Errors will still print because stderr isn't redirected.

("infinite scrolling" is hyperbole)

NOTE: This really only applies when custom-building Iosevka, because
otherwise it gets fetched from the cache. Still extremely annoying when
you have a private build plan like I do.

Comparison

with node spam (imagine this happening for minutes at a time):

without node spam (nice and calm!):

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.

[babariviere]

Tested locally, works great 👍