DRIVERS-3032 ensure uv is installed after venv activation

[eramongodb]

Followup to #591 and #591.

Several issues appear to be related to the errors that motivated prior PRs:

• If venv already exists, it may be activated without a call to pip install uv even if uv has not yet been installed into that venv.
• uv may not necessarily be installed directly into venv/bin or venv/Script. It is possible for the binary to reside in a deeper subdirectory such as "site-packages", which in turn may also be within a Python-versioned subdirectory, such as "python-3.12" (given uv is designed to support multiple Python versions). The pushd $1 after activating the virtual environment may additionally break $PATH expectations (although I would expect/assume calling pip or python -m pip within an activate environment should be resiliant to this...).

Regardless, it seems sufficent to 1) ensure uv is checked and installed after activation (regardless whether the environment already existed or not, and 2) store and use the full path to the resulting binary (via which) to account for changing working directories.

[blink1073]

Passing with python driver, merging, thanks!