Update badge to current CI status. Remove the link to `/actions` (#2612)
## Summary
Hi! I noticed the badge in the `README.md` says CI it's failing
[](https://github.com/astral-sh/uv/actions)
But current main CI is okay. The problem is that badge is bringing the
latest actions ran in the repo. Even if it's from a PR
So I changed to this link:

It shows the current main branch CI status, but _I couldn't get the link
to work (go into `uv/actions`)_. Is it acceptable to lose some
functionality in order to display the correct information?
## Test Plan
manually