chore(merge drivers): maybe fix yarn merge driver

by actually writing to %A, see
https://git-scm.com/docs/gitattributes#_defining_a_custom_merge_driver:~:text=The%20merge%20driver%20is%20expected%20to%20leave%20the%20result%20of%20the%20merge%20in%20the%20file%20named%20with%20%25A%20by%20overwriting%20it

also see https://github.com/charpeni/merge-drivers-cli/issues/7

.merge-drivers.yml

···
 merge-drivers:
   regen-yarn-lockfile:
     name: regenerate yarn.lock
-    driver: yarn install
+    driver: yarn install; cp yarn.lock %A