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 moves two directories into globally-accessible locations:

- The uv Python installation directory: /opt/python
- The global virtual environment: /opt/python-venv

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>