nix: add channel configuration and rework update process

nix/home-manager/profiles/nix-channels.nix

@@ -0,0 +1,34 @@
+{ pkgs
+, config
+, ...
+}:
+
+let
+in {
+  home.file.".nix-channels".text = ''
+    https://nixos.org/channels/nixos-18.09 nixos
+    https://nixos.org/channels/nixos-unstable nixos-unstable
+  '';
+
+  home.activation._removeExistingNixChannels = config.lib.dag.entryBefore ["checkLinkTargets"] ''
+    $DRY_RUN_CMD ${pkgs.writeScript "activation-script" ''
+    set -ex
+    if test -f $HOME/.nix-channels; then
+      echo Moving existing file away...
+      touch $HOME/.nix-channels.dummy
+      mv --backup=numbered $HOME/.nix-channels.dummy $HOME/.nix-channels
+      rm $HOME/.nix-channels
+    fi
+    ''};
+  '';
+
+  nixpkgs.config = {
+
+    packageOverrides = pkgs: with pkgs; {
+      homeshick = builtins.fetchGit {
+        url = "https://github.com/andsens/homeshick.git";
+        ref = "master";
+      };
+    };
+  };
+}