ci: use python3 for building imgtool wheel

Signed-off-by: Fabio Utzig <fabio.utzig@nordicsemi.no>
diff --git a/ci/imgtool_run.sh b/ci/imgtool_run.sh
index 62ea77e..20b5420 100755
--- a/ci/imgtool_run.sh
+++ b/ci/imgtool_run.sh
@@ -48,6 +48,6 @@
 fi
 
 rm -rf $DIST_DIR
-python setup.py sdist bdist_wheel
+python3 setup.py sdist bdist_wheel
 
 twine upload --username __token__ --password "${TWINE_TOKEN}" "${DIST_DIR}/*"