fix(python): scope checkout caches to the specific project

Since the template uses the `mozilla` trust domain, it's expected for
workers to run tasks for multiple unrelated projects. Therefore we don't
want to share a checkout cache between them (and especially not call
them all `src`).

This scopes the checkout cache to just the specific project as well as
renames it from `src` -> `${project}`.

reps/templates/python/{{cookiecutter.__project_slug}}/.taskcluster.yml

@@ -218,7 +218,7 @@ 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