pfalcon-tf-m-static: Try "refspec" param instead of "branches"

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: I22ba9a78c9d40de571d8491cf83a2ca80adbbbc0
diff --git a/pfalcon-tf-m-static.yaml b/pfalcon-tf-m-static.yaml
index 1c80e8e..53d905a 100644
--- a/pfalcon-tf-m-static.yaml
+++ b/pfalcon-tf-m-static.yaml
@@ -150,8 +150,7 @@
       scm:
         - git:
             url: '$CI_SCRIPTS_REPO'
-            branches:
-            - '$CI_SCRIPTS_BRANCH'
+            refspec: '$CI_SCRIPTS_BRANCH'
             credentials-id: GIT_SSH_KEY
             wipe-workspace: false
       script-path: jenkins/static.jpl