fix(copybara-base): correctly fill git credentials via jinja2 template

This is another tentative fix.

Change-Id: I2bb443873aa38822a33271eb6e676af96037eb7e
diff --git a/playbooks/copybara-base/pre.yaml b/playbooks/copybara-base/pre.yaml
index cd87fc7..00e77f3 100644
--- a/playbooks/copybara-base/pre.yaml
+++ b/playbooks/copybara-base/pre.yaml
@@ -1,5 +1,5 @@
 - hosts: all
-  vars:
-    git_credentials: copybara_secrets.git_credentials
   roles:
     - role: add-copybara-credential-store
+      vars:
+        git_credentials: "{{ copybara_secrets.git_credentials }}"