add a new updater output

this is so that we don't have to obfuscate store paths in
systemConfiguration to avoid dragging in build system
deps.

breaking-ish change to workflows, docs updated

modules/outputs/updater/default.nix

@@ -0,0 +1,36 @@
+{
+  config
+, pkgs
+, lib
+, ...
+}:
+let
+  inherit (lib) mkIf;
+  o = config.system.outputs;
+  inherit (pkgs) runCommand;
+  inherit (lib) mkOption types;
+  inherit (pkgs.buildPackages) min-copy-closure;
+in
+{
+  imports = [ ../system-configuration.nix ];
+  options.system.outputs.updater = mkOption {
+    type = types.package;
+    description = ''
+      updater
+      ******
+
+      For configurations with a writable filesystem, create a shell
+      script that runs on the build system and updates the device
+      over the network to the new configuration
+    '';
+  };
+
+  config.system.outputs.updater =
+    runCommand "buildUpdater" { } ''
+      mkdir -p $out/bin
+      substitute ${./update.sh} $out/bin/update.sh \
+         --subst-var-by toplevel ${o.systemConfiguration} \
+         --subst-var-by min_copy_closure ${min-copy-closure}
+      chmod +x  $out/bin/update.sh
+    '';
+}