Replace Hugo builder with inline scripts

[davidstanke]

Maintaining the custom Hugo builder image for Cloud Build adds complexity with limited benefit. Replace it with scripts that install necessary tools on the fly.

Also see #309

[davidstanke]

Hey @jessegduke ! FYI, I stopped using the Hugo builder and replaced it with inline installers in CI. (This will make it easier to add a svelte build step, which I need to do for the new quick check [see /experimental].) You can see the current state in the /ci/cloudbuild*.yaml configs. There is some duplication -- there are three different Cloud Build configs, each of which installs Hugo on the fly. As discussed, that's the right answer! But we might consider writing a little bash script to DRY out those installation steps(?)