CI: upgrade-compat: check if pmb is installed (MR 12)

.ci/upgrade-compat.sh

-#!/bin/sh -ex
+#!/bin/sh -e
 # Copyright 2023 Dylan Van Assche, Oliver Smith
 # SPDX-License-Identifier: GPL-3.0-or-later
 # Description: test upgrade.sh in pmb chroot (use env vars to set CHANNEL_OLD etc)
@@ -12,6 +12,13 @@ if [ "$(id -u)" = 0 ]; then
 	exec su "${TESTUSER:-pmos}" -c "sh -e $0"
 fi
 
+if [ -z "$(command -v pmbootstrap 2>/dev/null)" ]; then
+	echo "ERROR: install pmbootstrap first, consider using 'pmbootstrap ci'"
+	echo "       https://postmarketos.org/pmbootstrap"
+	exit 1
+fi
+
+set -x
 DEVICE="${DEVICE:-qemu-amd64}"
 UI="${UI:-none}"
 CHANNEL_OLD="${CHANNEL_OLD:-v22.12}"