Deployments with extra requirements

I’m using a hosted Kubernetes cluster with a local server and agents/workers with prefect-helm repo: (GitHub - PrefectHQ/prefect-helm: Helm charts for deploying Prefect Services) .
in my setup I want deployments to run ad-hoc with little overhead once they are triggered, so I’m going with subprocess option running on the agents.
some of my deployments have extra requirements (some general packages and some hosted on a private pypi server).

what is the easiest way of making sure that each worker has all the requirements for each Deployment I create?
looking at the options it would be great to use github/bitbucket as code storage but with that I don’t see an option to install requirements.

another question:
if I decide to use git as code storage, when does the pulling/requirements.txt install take place?
at agent startup?
when a flow run is triggered?