direnv: use nix print-dev-env

In order to increase determinism in the build environment, we are
having direnv load the shell environment from the flake's `devShell`
output, instead of the traditional `nix-shell` approach. Of course,
one can still enter a `nix-shell` manually on a system without flake
support installed.

.envrc

@@ -1 +1,5 @@
-use nix
+# reload when these files change
+watch_file flake.nix
+watch_file flake.lock
+# load the flake devShell
+eval "$(nix print-dev-env)"

flake.nix

@@ -40,6 +40,8 @@
           import ./hosts (inputs // { inherit system pkgs unstablePkgs; });
       in configs;
 
+      devShell."${system}" = import ./shell.nix { inherit pkgs; };
+
       overlay = import ./pkgs;
 
       overlays = let