CI: shellcheck: check if it is installed (MR 12)

.ci/shellcheck.sh

@@ -13,6 +13,11 @@ if [ "$(id -u)" = 0 ]; then
 	exec su "${TESTUSER:-build}" -c "sh -e $0"
 fi
 
+if [ -z "$(command -v shellcheck 2>/dev/null)" ]; then
+	echo "ERROR: install shellcheck or use 'pmbootstrap ci'"
+	exit 1
+fi
+
 # Shell: shellcheck
 sh_files="
 	$(find . -path '*.sh')