ci: use /usr/bin/env instead of /bin/env

Signed-off-by: Paul Meyer <49727155+katexochen@users.noreply.github.com>

image/mkosi.postinst

@@ -1,4 +1,4 @@
-#!/bin/env bash
+#!/usr/bin/env bash
 set -euxo pipefail
 
 # This will work in sd-boot 251 to auto-enroll secure boot keys.

image/mkosi.prepare

@@ -1,4 +1,4 @@
-#!/bin/env bash
+#!/usr/bin/env bash
 set -euxo pipefail
 
 # set selinux to permissive