fix: pass --break-system-packages for uv-managed Python (PEP 668) (#3073)
GPU builds on nvidia/cuda base images install Python via `uv python
install`, which marks it as externally managed. uv 0.9.x enforces
PEP 668, so pip refuses to install into that environment without
--break-system-packages.
The flag was already wired up behind SetBreakSystemPackages, but only
the `cog base-image` CLI command called it. Normal `cog build` never
did, breaking every GPU build that uses the CUDA base image path.