Revert "ci/kconfig.sh: use pmbootstrap v3 (MR 5301)" (MR 5658)

See parent commit.

This reverts commit c8e2ad0b.
[ci:skip-build]: already built successfully in CI

.ci/kconfig.sh

@@ -6,7 +6,6 @@
 
 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"