Use uv and caching in CI

[galenlynch]

Should speed things up!

[samuelgarcia]

very cool.
why do we need the --system flag ? for caching ?

[galenlynch]

So uv uses the setup-python-installed python and installs packages into that, instead of trying to use its own python and venv. That allows python args... commands to 'just work', instead of needing to do uv run python args.... Since not all maintainers are using uv lockfiles etc, I think this is the simplest way to use uv pip as a faster pip, instead of having to adopt uv's entire workflow.

Python is also cached on many of the github runners, so setup-python is quite a bit faster (it's already on the runner) than using uv to get python (download over network).

[galenlynch]

Put another way, pip install already installs into the system site packages if it's not run in a virtual environment. In contrast, uv pip install will install into a uv-maintained virtual environment even if it's not run in a virtual environment. To truly emulate pip install with uv, you need to tell uv you really want to mutate the system site packages with uv pip install --system, or set an equivalent environment variable (but that affects all other uv commands, which might not be what you want).