SRE-8406: Have Pluto Github workflow grab dependencies from jfrog

[cttttt]

Re: https://thescore.atlassian.net/browse/SRE-8406

During Pluto checks, fetches helm chart dependencies prior to rendering helm template. This is now required since some helm charts depend on a core SRERE helm chart, stored in our private jfrog repo. Although jfrog is accessible from the internet, its credentials live in Vault, which is not reachable from the internet.

Changes:

• Uses SRERE's handy action to authenticate with jfrog.
• This action requires Vault access. Since vault is inaccessible from the internet, uses our internal Github runners.
• Unfortunately replaces an upstream action to install Pluto. This action depends on podman, which is not available from our Github Actions self-hosted runners (we use Docker as our container runtime).
• Fetches helm dependencies prior to trying to render a template.

How do I know this works?

See the before and after in this workflow run: link.

This run was invoked from this branch of scorebet/edgebook-manifests. In this branch, I modified just the edgebook pluto check workflow invocation to use this pull request branch's version of the workflow.

Notes for reviewers:

Github workflows are myserious and interesting. Review commit by commit to track my journey.