fix: restore _BODY substitution to Pub/Sub triggered builds

[grayside]

This was mostly removed over the course of various refactoring, but recent troubleshooting needs show it's value.

• Use the entire Pub/Sub body as a substitution
• Echo this object to the build log as the first step (before anything else can go wrong)
• Re-order substitutions in terraform config to alphabetical order to match display order in console.