Rewrite ghcr.io in devcontainer features to use the pull through cache

[pantherman594]

Use our pull through cache for devcontainer features so we don't run into ghcr.io's aggressive rate limiting.

We should do this at runtime after pulling the git repo, rather than saving them to this repository, since it could muddy dependency order resolution if other features are added. For example, our workbench-tools feature has

  "installsAfter": [
    // ...
    "ghcr.io/devcontainers/features/conda",
    // ...
  ]

if we rewrote this to

  "installsAfter": [
    // ...
    "us-central1-docker.pkg.dev/workbench-app-cache/ghcr-cache/devcontainers/features/conda",
    // ...
  ]

and someone added ghcr.io/devcontainers/features/conda to their custom app, workbench-tools could potentially get installed before conda since installsAfter matches the feature by repository URL.