Check if actions already exist in feeds.post_update.php

feeds.post_update.php is not compatible with certain deployment workflows. For example:

LOCAL (tasks run manually):
1. composer update
2. drush updb
3. drush config:export
4. commit changes to git and push to dev repository

DEV deployment triggered by push from local (tasks run automatically):
1. composer install -y
2. drush cr
3. drush config:import -y
4. drush updb -y

In this example drush updb fails on the dev environment because the feeds_feed_clear_action and feeds_feed_import_action have already been created by the configuration import in the previous step.

Suggest to add a check into the update to first see if the actions already exist.

Many thanks.