install-nix-action

• BREAKING: By default it has no nixpkgs configured, you have to set nix_path
  by picking a channel
  or pin nixpkgs yourself.
• Support self-hosted runners by not installing Nix if it's already installed
• Add extra_nix_config option to specify extra nix.conf options

Allow setting nix_path

• allow specifying install_url, mostly useful for testing non-stable Nix releases
• allow skipping installation of nixpkgs channel via skip_adding_nixpkgs_channel: true to speed up installation for ~5s
• a bunch of improvements in Nix 2.3.5 significantly speeds up installation: Linux can now install in ~7s, macOS in ~20s

Allow installation path to redirect

• macos: bootstrap Nix with the new volume approach, ditching symlink that had issues like #21
• macos: reliably wait for nix-daemon socket connection
• macos: disable spotlight for /nix, speeding up builds
• rewrite most of the script to bash, so there's no more need for node_modules

Enable daemon (multi-user) installation of Nix for macOS and Linux.
Enable sandbox for Linux.

The downside of this check is that the installation of Nix takes much longer, around 1min on macOS.

This can be improved with #13
macOS sandbox support tracked at #15

v4 was a brown bag release.

Fix macOS (Catalina) support.

• speed up builds by setting max-jobs = auto in /etc/nix/nix.conf
• properly avoid segfault when installing Nix

• sync javascript with typescript (and add a CI check for it)
• include node_modules to fix action