[#210] Add activation script for darwin system and provide a usage example

Problem: It's possible to use 'deploy-rs' for deploying 'darwinSystem'
configuration from 'nix-darwin' to a darwin system. However, there is no
dedicated activatiot script for darwin and thus one has to come up with
'custom' activation script.

Solution:
1) Add 'darwin' attribute to 'lib.activate' that provides a script that
   should be used to activate 'darwinSystem' config with 'deploy-rs'.
2) Add a new 'examples/darwin' example that provides simple flake for
   deploying configuration to a darwin target.

1 files changed, 7 insertions, 0 deletions

diff --git a/flake.nix b/flake.nix
index 9b8e6eb..0616cf8 100644
--- a/flake.nix
+++ b/flake.nix
@@ -106,6 +106,13 @@
 
             home-manager = base: custom base.activationPackage "$PROFILE/activate";
 
+            # Activation script for 'darwinSystem' from nix-darwin.
+            # 'HOME=/var/root' is needed because 'sudo' on darwin doesn't change 'HOME' directory,
+            # while 'darwin-rebuild' (which is invoked under the hood) performs some nix-channel
+            # checks that rely on 'HOME'. As a result, if 'sshUser' is different from root,
+            # deployment may fail without explicit 'HOME' redefinition.
+            darwin = base: custom base.config.system.build.toplevel "HOME=/var/root $PROFILE/activate";
+
             noop = base: custom base ":";
           };