Use a stable directory for (local) script virtual environments (#11347)

## Summary

Today, scripts use `CachedEnvironment`, which results in a different
virtual environment path every time the interpreter changes _or_ the
project requirements change. This makes it impossible to provide users
with a stable path to the script that they can use for (e.g.) directing
their editor.

This PR modifies `uv run` to use a stable path for local scripts (we
continue to use `CachedEnvironment` for remote scripts and scripts from
`stdin`). The logic now looks a lot more like it does for projects: we
`get_or_init` an environment, etc.

For now, the path to the script is like:
`environments-v1/4485801245a4732f`, where `4485801245a4732f` is a SHA of
the absolute path to the script. But I'm not picky on that :)

crates/uv-scripts/src/lib.rs

@@ -65,7 +65,7 @@ impl Pep723Item {
 }
 
 /// A reference to a PEP 723 item.
-#[derive(Debug)]
+#[derive(Debug, Copy, Clone)]
 pub enum Pep723ItemRef<'item> {
     /// A PEP 723 script read from disk.
     Script(&'item Pep723Script),