Update GHA workflow to workaround bug in third-party action

[eecavanna]

On this branch, we updated our "CI" GHA workflow in two ways:

• Add a workaround for a bug in the GHA that installed uv
• Removed a third-party GHA that spins up the Docker Compose stack (we'll spin it up manually instead)
• Updated the Mongo container image version

[eecavanna]

FYI @jeff-cohere, while trying to debug failing GHA workflow runs, we decided to run docker compose up directly instead of via a third-party GHA.

[eecavanna]

Reviewed by @shreddd over Zoom (more or less).