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

pkgs/systemconfig/build-system-install.sh

@@ -1,53 +0,0 @@
-#!/usr/bin/env bash
-
-# this shell script can be run on the build system to
-# min-copy-closure the system configuration onto the device
-# and reboot/restart services as requested
-
-die() {
-    echo "$@"
-    exit 1
-}
-
-# add min-copy-closure to the path. removing junk characters
-# inserted by default.nix (q.v.)
-min_copy_closure=@min-copy-closure@; PATH=${min_copy_closure//_/}/bin:$PATH
-
-ssh_command=${SSH_COMMAND-ssh}
-
-reboot="reboot"
-
-case "$1" in
-    "--no-reboot")
-	unset reboot
-	shift
-	;;
-    "--fast")
-	reboot="soft"
-	shift
-	;;
-esac
-
-target_host=$1
-shift
-
-test -n "$target_host" || \
-    die "Usage: $0 [--no-reboot] [--fast] target-host"
-
-toplevel=$(cd -- "$( dirname -- "${BASH_SOURCE[0]}" )" &> /dev/null && readlink `pwd` )
-test -e $toplevel/etc/nix-store-paths || die "missing etc/nix-store-paths, is this really a system configuration?"
-echo installing from systemConfiguration $toplevel to host $target_host
-
-min-copy-closure $target_host $toplevel
-set -x
-$ssh_command $target_host $toplevel/bin/install
-case "$reboot" in
-    reboot)
-	$ssh_command $target_host "sync; source /etc/profile; reboot"
-	;;
-    soft)
-	$ssh_command $target_host $toplevel/bin/restart-services
-	;;
-    *)
-	;;
-esac

pkgs/systemconfig/default.nix

@@ -83,11 +83,6 @@ in attrset:
       $STRIP --remove-section=.note  --remove-section=.comment --strip-all activate -o $out/bin/activate
       ln -s ${s6-init-bin}/bin/init $out/bin/init
       cp -p ${writeFennel "restart-services" {} ./restart-services.fnl} $out/bin/restart-services
-      # obfuscate the store path of min-copy-closure so that the output
-      # closure doesn't include a bunch of build system stuff
-      f=${buildPackages.min-copy-closure}; f=$(echo $f | sed 's/\(.....\)/\1_/g')
-      substitute ${./build-system-install.sh} $out/install.sh --subst-var-by min-copy-closure $f
-      chmod +x $out/install.sh
       cat > $out/bin/install <<EOF
       #!/bin/sh -e
       prefix=\''${1-/}