Use nixpkgs-2105 from out checkout instead of passing a path around

This is what we wanted and much more directly.  It also avoids the gradual
lengthening of the path by always setting the path to nixpkgs-2105 instead of
some derivative of whatever the path was last time.

nixos/modules/deployment.nix

@@ -11,13 +11,7 @@ let
     # `restrict` means "disable all the things" then `command` means "but
     # enable running this one command" (the client does not have to supply the
     # command; if they authenticate, this is the command that will run).
-    # environment lets us pass an environment variable into the process
-    # started by the given command.  It only works because we configured our
-    # sshd to allow this particular variable through.  By passing this value,
-    # we can pin nixpkgs in the executed command to the same version
-    # configured for use here.  It might be better if we just had a channel
-    # the system could be configured with ... but we don't at the moment.
-    "restrict,environment=\"NIXPKGS_FOR_MORPH=${pkgs.path}\",command=\"${command} ${gridName}\" ${authorizedKey}";
+    "restrict,command=\"${command} ${gridName}\" ${authorizedKey}";
 in {
   options = {
     services.private-storage.deployment.authorizedKey = lib.mkOption {
@@ -50,10 +44,6 @@ in {
       ];
     };
 
-    services.openssh.extraConfig = ''
-      PermitUserEnvironment=NIXPKGS_FOR_MORPH
-    '';
-
     # Create a one-time service that will set up an ssh key that allows the
     # deployment user to authorize as root to perform the system update with
     # `morph deploy`.

nixos/modules/update-deployment

@@ -75,12 +75,8 @@ EOF
 # Make sure known_hosts has the host key in it.
 ssh -o StrictHostKeyChecking=no "$(hostname).$(domainname)" ":"
 
-# Set nixpkgs to our preferred version for the morph build.  The NIX_PATH
-# environment variable itself receives special treatment by some parts of the
-# system (especially those parts leading up to the execution of this script)
-# so we pass the desired information through a different variable and then
-# shuffle it into the right place here, just before it is needed.
-export NIX_PATH="nixpkgs=$NIXPKGS_FOR_MORPH"
+# Set nixpkgs to our preferred version for the morph build.
+export NIX_PATH="nixpkgs=${CHECKOUT}/nixpkgs-2105.nix"
 
 # Attempt to update just this host.  Choose the morph grid definition matching
 # the grid we belong to and limit the morph deployment update to the host