UV has no cache dir if init container runs as non-1000 UID

[dan-fernandes]

The init container uses UV to obtain python dependencies. This will cause UV to set up a cache dir, which defaults to this hierarchy:

1. A temporary cache directory, if --no-cache was requested.
2. The specific cache directory specified via --cache-dir, UV_CACHE_DIR, or tool.uv.cache-dir.
3. A system-appropriate cache directory, e.g., $XDG_CACHE_HOME/uv or $HOME/.cache/uv on Unix and %LOCALAPPDATA%\uv\cache on Windows

With the current setup this defaults to $HOME/.cache/uv which if running as user 1000 resolves to /home/ubuntu/.cache/uv. If running as any other UID (e.g. 37889 for p49detector) the $HOME envar will not exist and UV will fail.

Acceptance Criteria

• When a UID other than 1000 is used, the init container can successfully install all scratch dependencies.

[dan-fernandes]

Before the holidays I was developing a solution into this branch of BlueAPI: https://github.com/DiamondLightSource/blueapi/tree/fix-uv-cache-dir