use Git as source for bitnami Helm charts

Related to #1947 (closed)

We may eventually consider using Bitnami OCI artifacts as the upstream source used in Sylva, but in the meantime this MR changes our sources to use the Git repository instead.

Even if each FluxCD GitRepository resource will result in a checkout of the whole github.com/bitnami/charts.git Git repo, thanks to shallow cloning, each will take only approximately 8MB.

In this MR quite a few adaptations where necessary to the code we have to generate OCI artifacts:

• need to adapt to the <tool>/<version> tags used in the github.com/bitnami/charts.git Git repo
• need to not run always run helm dependency update (because it updates Chart.lock resulting in different artifacts that the one that had been published), and instead run helm dependency build (first, but then fallback to helm dependency update if this fails because Chart.lock is incomplete or inexisting)
• in the process I realized that the Python code in tools/oci was lacking error checks when running commands with subprocess.run calls; I added a helper to clean this up and simplify the code everywhere (also did other minor improvements)

💡 This MR will be more easily reviewed one commit at a time