ci/pytest: make sure tests run on correct branch (MR 6418)

This works around a situation where pmbootstrap init checks out the
wrong branch and we end up executing the wrong version of the tests.

For context:
https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/6418#note_477266

.ci/pytest.sh

@@ -10,6 +10,8 @@ if [ "$(id -u)" = 0 ]; then
 	set -x
 	wget "https://gitlab.postmarketos.org/postmarketOS/ci-common/-/raw/master/install_pmbootstrap.sh"
 	sh ./install_pmbootstrap.sh pytest
+	# FIXME: workaround for pmb init switching the branch on us...
+	su "${TESTUSER:-pmos}" -c "git -C $CI_PROJECT_DIR checkout $CI_COMMIT_SHA"
 	exec su "${TESTUSER:-pmos}" -c "sh -e $0"
 fi