Legacy explorer frontend dependency on NodeJS 6.x

[deepfire]

After bumping haskell.nix/iohk-nix/Nixpkgs, we no longer have the 6.x package set for NodeJS: https://buildkite.com/input-output-hk/cardano-sl/builds/13937#_

explorerFrontend: attribute 'nodejs-6_x' missing, at /var/lib/buildkite-agent/builds/packet-buildkite-3-ci3-1/input-output-hk/cardano-sl/explorer/frontend/default.nix:63:14

This can potentially be pretty bad, if:

1. We can't avoid using new haskell.nix, and
2. nodejs-10_x doesn't do

[deepfire]

Blocks #4252