home-manager: simplify use of nix-build output
home-manager: simplify use of nix-build output

There is no need to specify an out link when switching to a new
generation since nix-build prints the store path on standard output.
Similarly, when just building a generation we specify no out link
since nix-build will use "result" by default.

rycee/home-manager,rycee/home-manager,rycee/home-manager