build-support/setup-hooks: change shebang to shellcheck directive

pkgs/build-support/setup-hooks/auto-patchelf.sh

···
-#!/usr/bin/env bash
+# shellcheck shell=bash
 
 declare -a autoPatchelfLibs
 declare -a extraAutoPatchelfLibs

pkgs/build-support/setup-hooks/desktop-to-darwin-bundle.sh

···
-#!/usr/bin/env bash
+# shellcheck shell=bash
 fixupOutputHooks+=('convertDesktopFiles $prefix')
 
 # Get a param out of a desktop file. First parameter is the file and the second

pkgs/build-support/setup-hooks/move-systemd-user-units.sh

···
-#!/usr/bin/env bash
+# shellcheck shell=bash
 
 # This setup hook, for each output, moves everything in
 # $output/lib/systemd/user to $output/share/systemd/user, and replaces