.gitlab-ci.yml: Use 0.15.0 tag on overnigth tests
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 09dce18..9922ee0 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -98,7 +98,7 @@
   stage: test
   variables:
     bst_ext_url: git+https://gitlab.com/BuildStream/bst-external.git
-    bst_ext_ref: 0.9.0-0-g63a19e8068bd777bd9cd59b1a9442f9749ea5a85
+    bst_ext_ref: 0.15.0-0-g4915bda740e628f35d55d60d27523492c0ee7be7
     fd_sdk_ref: freedesktop-sdk-18.08.34-0-g9282304d0d932032b8297fb049f44d6a079dc2ea
   before_script:
   - pip3 install -r requirements/requirements.txt -r requirements/plugin-requirements.txt