Fixes coder/internal#706

Context for the implementation here
coder/internal#706 (comment)

Synchronously starts dev containers defined in terraform with our
`DevcontainerCLI` abstraction, instead of piggybacking off of our
`agentscripts` package. This gives us more control over logs, instead of
being reliant on packages which may or may not exist in the
user-provided image.

Updates CI job definitions to cache downloaded binaries for embedded-postgres.

…18568)

…n tests (#18570)