Automatically use uv instead of pip if it is available.

[tylerjthomas9]

pip sometimes takes a long time to resolve dependencies, so I added support for uv (https://github.com/astral-sh/uv). CondaPkg.jl will automatically use uv instead of pip if uv is present in the environment. uv is designed as a drop-in replacement for pip, but I didn't want to start installing it by default on every process that pip installs libraries.

[cjdoris]

Thanks for this - I have just merged a tweaked version in #139