Make the cache dir wherever uv is called from when --no-cache specified

[shaneikennedy]

Summary

As brought up here, using the tempfile::env::temp_dir default per system is not ideal for cases where directories are on different mounts, or simply in the context where the system user doesn't have write permissions the temp_dir default.

I read the initial issue and @charliermarsh 's follow up comment and I think the coupling to the virtual env directory is not necessary, and instead I think we can simply default the tempdir to the directory where uv is being invoked from. I think this is a sensible default since it is common to run the project from the project root AND the .venv directory is created in the project root so we should have write permissions here.

In case for some reason the directory that uv is invoked from also doesn't have write permissions (or you just don't want the temp cache there for some reason?), I added a commit that will respect UV_CACHE_DIR so that these users can provide a directory for a temp cache when using --no-cache

Test Plan

uv git:main*
❯ UV_CACHE_DIR="/Users/shane.kennedy/dev/shane/" target/debug/uv cache dir --no-cache
/Users/shane.kennedy/dev/shane/.tmpfBmOPp

uv git:main*
❯ target/debug/uv cache dir --no-cache
/Users/shane.kennedy/dev/shane/uv/.tmp4b6zap

Also used the uv build for building/syncing some test projects and it seems to work fine, I think the test suite for uv will be more comprehensive, however.

[shaneikennedy]

@edmorley Hey :) Since this PR attempts to cover your use-case in the issue you submitted re:--no-cache, would you mind taking a look at the PR description and letting me know if you think this would resolve the issue for you at Heroku, please? I think I understood your problem correctly but want to double check

I know your suggestion was to put the temp dir inside the venv but I understood that the main problem was hard linking and not wanting to do manual cleanup, which should be satisfiable here either by the default (temp dir inside directory where uv is invoked from) or by using UV_CACHE to create the temp dir somewhere that works for you