ci/kconfig.sh: use pmbootstrap v3 (MR 5301)

Use pmbootstrap v3 for kconfig check, which reads kconfigcheck.toml from
the current pmaports branch and uses it.

.ci/kconfig.sh

@@ -6,6 +6,7 @@
 
 if [ "$(id -u)" = 0 ]; then
 	set -x
+	export PMBOOTSTRAP_TAG=master
 	wget "https://gitlab.com/postmarketOS/ci-common/-/raw/master/install_pmbootstrap.sh"
 	sh ./install_pmbootstrap.sh
 	exec su "${TESTUSER:-pmos}" -c "sh -e $0"