Apply advisory locks when building source distributions (#3525)
## Summary
I don't love this, but it turns out that setuptools is not robust to
parallel builds: https://github.com/pypa/setuptools/issues/3119. As a
result, if you run uv from multiple processes, and they each attempt to
build the same source distribution, you can hit failures.
This PR applies an advisory lock to the source distribution directory.
We apply it unconditionally, even if we ultimately find something in the
cache and _don't_ do a build, which helps ensure that we only build the
distribution once (and wait for that build to complete) rather than
kicking off builds from each thread.
Closes https://github.com/astral-sh/uv/issues/3512.
## Test Plan
Ran:
```sh
#!/bin/bash
make_venv(){
target/debug/uv venv $1
source $1/bin/activate
target/debug/uv pip install opentracing --no-deps --verbose
}
for i in {1..8}
do
make_venv ./$1/$i &
done
```
