This PR is the second in a series aimed at closing
https://github.com/coder/coder/issues/15109.
## Changes
- adds `scripts/embedded-pg/main.go`, which can start a native Postgres
database. This is used to set up PG on Windows and macOS, as these
platforms don't support Docker in Github Actions.
- runs the `test-go-pg` job on macOS and Windows too
- adds the `test-go-race-go` job, which runs race tests with Postgres on
Linux
