fix(copybara-base): pass git credentials correctly

This is a tentative fix.

Change-Id: I089740e61bb0cee486dff8d3e4f7cff6bda76114
diff --git a/playbooks/copybara-base/pre.yaml b/playbooks/copybara-base/pre.yaml
index 4c4f314..cd87fc7 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