Integrate Galileo deployment with CD

[zbuc]

• [ ] Create galileo-preview deployment to run against the main branch of penumbra and testnet-preview. Perhaps send messages to a different private channel or disable Discord messaging entirely.
• [ ] Modify testnet-preview GitHub Action to automatically deploy galileo-preview on main branch
• [ ] Modify testnet deploy GitHub Action to automatically deploy galileo on testnet release tag
• [ ] Update testnet deployment template to incorporate any changes to process

The trickiest thing is that we need to pin the dependency versions of the penumbra repos within the Galileo Cargo.toml.