Support UV_PROJECT environment to set project directory. (#12327)

## Summary

<!-- What's the purpose of the change? What does it do, and why? -->
Support the `UV_PROJECT` environment variable to set the project
directory.
#11946 

## Test Plan

<!-- How was it tested? -->
`cargo nextest run` passed except the cache_prune.
```
export UV_PROJECT=/path/to/project
uv sync
```
works.

Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>

crates/uv-cli/src/lib.rs

@@ -319,7 +319,7 @@ pub struct GlobalArgs {
     /// See `--directory` to change the working directory entirely.
     ///
     /// This setting has no effect when used in the `uv pip` interface.
-    #[arg(global = true, long)]
+    #[arg(global = true, long, env = EnvVars::UV_PROJECT)]
     pub project: Option<PathBuf>,
 }