ci: fix virtual environment configuration

Shrinkwrap does not always run its build containers as the root user, which renders the virtual environment installed within the root user's home directory inaccessible.

This change instead installs the Python virtual environment to a global directory - /opt/python - to make installed Python binaries accessible to non-root users.

This fixes an additional issue where uv was also not syncing to the correct virtual environment to begin with, and was instead trying to sync a new project environment in /shrinkwrap.

Signed-off-by: Chris Kay chris.kay@arm.com