chore: rename duplicate CircleCI variable (#1141)

Strangely, CircleCI works on this repo but does not work on a cloned
repo because of a duplicate variable name. Fixes #1138.

.circleci/config.yml

diff --git a/.circleci/config.yml b/.circleci/config.yml
index c1021db..e019354 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -88,7 +88,7 @@
     keys:
       - *cache_key
 
-var_8: &attach_workspace
+var_11: &attach_workspace
   attach_workspace:
     at: ~/