Link /Applications to ~/Applications/Nix\ Darwin

[supermarin]

Step 1: try to resolve conflicts between nix-darwin and home-manager regarding linking in ~/Applications. Currently, home-manager has disabled linking to ~/Applications because nix-darwin is behaving inconsistently and sometimes creating a race:

• If there's no ~/Applications folder on the system, nix-darwin will symlink it's apps directly to ~/Applications. This is bad because home-manager can't symlink under ~/Applications/Home Manager Apps
• If there is ~/Applications folder already on the system, nix-darwin will symlink into ~/Applications/Nix Apps.

This PR ensures that there's an ~/Applications folder, and always uses ~/Applications/Nix Apps for symlinks.

Related issue: https://github.com/nix-community/home-manager/issues/1341

[toonn]

I opened #470 for this, takes care of pre-existing ~/Applications symlinks managed by nix-darwin and such too, only needs testing.

[supermarin]

Thanks @toonn