fix(init): use project specific checkout cache by default

While some trust domains (like Gecko) might want to share checkout
caches across projects, typically projects are not related code bases to
one another. So we probably don't want to share checkout caches by
default.

template/{{cookiecutter.project_name}}/taskcluster.github.yml

@@ -198,7 +198,7 @@ tasks:
                                     ACTION_CALLBACK: '${action.cb_name}'
 
                       cache:
-                          "${trustDomain}-level-${level}-checkouts-sparse-v2": /builds/worker/checkouts
+                          "${trustDomain}-project-${project}-level-${level}-checkouts-sparse-v2": /builds/worker/checkouts
 
                       features:
                           taskclusterProxy: true