Fix nix symlink in MacOS after system upgrade

The MacOS system upgrades basiclly will uninstall Nix every time and reinstall it again. It will remove the symlink /run/current-system which is the basic environment you are using.

So we need to restore the symlink and activate the nix environment.

1. Find what’s the /run/current-system points to. Actually you may don’t have time to know this before your first run into this trouble, so my method is:

   • It usually points to one folder listed in /nix/store/, and with the string darwin-system in the folder name.
   • We can just use the most recent one as current system. In my case, I only have 1 recent system and others are all at 1970. It is easy to identify.

2. Link /run/current-system to the folder you found.

3. Then cd into the folder, and run ./activate to activate the system.