Add option pass environment variables for SDist building (#2039)

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## Summary

With this PR I've added the option environment variables to the wheel
building process, through the `BuildDispatch`. When integrating uv with
our project pixi (https://github.com/prefix-dev/pixi/pull/863). We ran
into this missing requirement, I've made a rough version here, could
maybe use some refinement.

### Why do we need this?

Because pixi allow the user to use a conda activated prefix for wheel
building, this comes with a number of environment variables, like `PATH`
but also `CONDA_PREFIX` amongst others. This allows the user to use
system dependencies from conda-forge to use during an sdist build.
Because we use `uv` as a library we need to pass in the options
programatically. Additionally, in general there is nothing holding a
python sdist back from actually depending on an environment variable,
see
e.g the test package: https://pypi.org/project/env-test-package/

### What about `ConfigSettings`

I think `ConfigSettings` does not suffice because e.g. CMake could
function differently when the `CONDA_PREFIX` is set. Also, we do not
know if the user supplied backend actually support these settings.

### Path handling

Because the user can now also supply a PATH in the environment map, the
logic I had was the following, I format the path so that it has the
following precedence

1. venv scripts dir.
2. user supplied path.
3. system path.

### Improvements

There is some path modification and copying happening everytime we use
the `run_python_script` function, I think we could improve this but
would like some pointers where to best put the maybe split and cached
version, we might also want to use some types to split these things up.


### Finally

I did not add any of these options to the uv executables, I first would
like to know if this is a direction we would want to go in. I'm happy to
do this or make any changes that you feel would benefit this project.

Also tagging @wolfv to keep track of this as well.  

## Test Plan

<!-- How was it tested? -->

---------

Co-authored-by: konsti <konstin@mailbox.org>

crates/uv-dev/src/build.rs

@@ -7,6 +7,7 @@ use fs_err as fs;
 
 use distribution_types::IndexLocations;
 use platform_host::Platform;
+use rustc_hash::FxHashMap;
 use uv_build::{SourceBuild, SourceBuildContext};
 use uv_cache::{Cache, CacheArgs};
 use uv_client::{FlatIndex, RegistryClientBuilder};
@@ -87,6 +88,7 @@ pub(crate) async fn build(args: BuildArgs) -> Result<PathBuf> {
         setup_py,
         config_settings.clone(),
         build_kind,
+        FxHashMap::default(),
     )
     .await?;
     Ok(wheel_dir.join(builder.build(&wheel_dir).await?))