ci: free up resources in docfx build (#12274)

Similar to #12273,
we also need to free up resources in the docfx build.


I ran the `docfx` build locally, and can confirm that the resources need
to be cleaned up as well. To confirm,
- delete .gitignore file in the root of the repo
- delete .gitignore file in the root of a package
- run `nox -s docfx` in the root of a package
- see the resources below that need to be cleaned up to free up space
 ```
        __pycache__/
        docs/__pycache__/
        docs/_build/
        google/cloud/accessapproval_v1/__pycache__/
        google/cloud/accessapproval_v1/services/__pycache__/
google/cloud/accessapproval_v1/services/access_approval/__pycache__/

google/cloud/accessapproval_v1/services/access_approval/transports/__pycache__/
        google/cloud/accessapproval_v1/types/__pycache__/
```

Author: parthea

ci/run_single_test.sh

@@ -59,6 +59,11 @@ case ${TEST_TYPE} in
         ;;
     docfx)
         nox -s docfx
+        # Clean up built docs and python cache after the build process to avoid
+        # `[Errno 28] No space left on device`
+        # See https://github.com/googleapis/google-cloud-python/issues/12271
+        rm -rf docs/_build
+        find . | grep -E "(__pycache__)" | xargs rm -rf
         retval=$?
         ;;
     prerelease)