Tweak fast build cache and file exposure (#2188)
* Clean up build cache
* Replace broken global .cog_cache with id=monobase-cache
* APT cache is not needed now that we build APT tarball with docker run
* Rename tmpMount variable name
* Only overwrite env.txt if different
* Bind mount . => /src during monobase.user build
* So that Python wheel or zip files in requirements.txt are exposed to uv pip install
* Fix tests
