fix(copybara-base): pass git credentials correctly

Third tentative fix.

Change-Id: Iecbaf6dc4ac930456cdf33bdc3b3496384b0f32d
diff --git a/playbooks/copybara-base/pre.yaml b/playbooks/copybara-base/pre.yaml
index 00e77f3..f2c2e13 100644
--- a/playbooks/copybara-base/pre.yaml
+++ b/playbooks/copybara-base/pre.yaml
@@ -1,5 +1,4 @@
 - hosts: all
   roles:
     - role: add-copybara-credential-store
-      vars:
-        git_credentials: "{{ copybara_secrets.git_credentials }}"
+      git_credentials: "{{ copybara_secrets.git_credentials }}"