Add option to disable automatically downloading uv

[jonancm]

Motivation

When prek runs, it will search for uv and, if uv is not present in the system, prek will automatically download and install it.

In some scenarios, it's forbidden by policy or not desirable to make connections to the internet and install arbitrary software on the system, such us in CI pipelines, where security and reproducibility are critical. In these cases, it's more desirable to get an error message when uv is not installed.

uv provides the python-downloads setting and the UV_PYTHON_DOWNLOADS environment variable, which can disable automatic Python downloads.

Proposal

prek should also provide a way to disable automatic uv downloads for those scenarios where installing arbitrary software from the internet is not desirable. Automatic downloads should be opt-in.

Implementation idea

The environment variable PREK_UV_SOURCE already enables users to select the download source for uv, but it assumes that users will always want this to happen automatically.

A new value could be added to PREK_UV_SOURCE to disable automatic downloads, such us none or disable, or the behavior of an empty value could be changed to disable.

Alternatively, a separate environment variable could be added, e.g. PREK_UV_DOWNLOADS, with similar semantics as UV_PYTHON_DOWNLOADS / python-downloads.

Ideally, prek should also support an analogous configuration option in the prek configuration file.