nixbuild/nix-quick-install-action@v2

Changes

• Fix Nix configuration so that it also works for older releases of Nix.

• Setup a proper Nix user profile so that tools like nix-env works as
  expected.

• Add CI tests verifying that the Cachix action (cachix/cachix-action) works
  with this action.

Supported Nix Versions

• 3.0pre20200829_f156513
• 2.3.7
• 2.2.2
• 2.1.3