Handle sigterm calls, fixes #6724 (#8933)
<!--
Thank you for contributing to uv! To help us out with reviewing, please
consider the following:
- Does this pull request include a summary of the change? (See below.)
- Does this pull request include a descriptive title?
- Does this pull request include references to any relevant issues?
-->
## Summary
This PR builds off of https://github.com/astral-sh/uv/pull/6738 to fix
#6724 (sorry for the new PR @charliermarsh I didn't want to push to your
branch, not even sure if I could). The reason the original PR doesn't
fix the issue described in #6724 is because the fastapi is ran in the
project context (as I assume a lot of use cases are). This PR adds an
extra commit to handle the signals in the project/run.rs file
~It also addresses the comment
[here](https://github.com/astral-sh/uv/pull/6738/files#r1734757548) to
not use the tokio ctrl-c method since we are now handling SIGINT
ourselves~ update, tokio handles SIGINT in a platform agnostic way,
intercepting this ouselves makes the logic more complicated with
windows, decided to leave the tokio ctrl-c handler
~[This
comment](https://github.com/astral-sh/uv/pull/6738/files#r1743510140)
remains unaddressed, however, the Child process does not have any other
methods besides kill() so I don't see how we can "preserve" the
interrupt call :/ I tried looking around but no luck.~ updated, this PR
is reduced to only handling SIGTERM propagation on unix machines, and
the sigterm call to the child is preserved by making use of the nix
package, instead of relying on tokio which only allowed for `kill()` on
a child process
## Test Plan
I tested this by building the docker container locally with these
changes and tagging it "myuv", and then using that as the base image in
uv-docker-example, (and ofc following the rest of the repro issues in
#6724. In my tests I see that ctrl-c in the docker-compose up command
exits the process almost immediately 👍
---------
Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>
