Create ephemeral virtual environments for uv run (#3075)

If a virtual environment does not exist, we will create one for the
duration of the invocation.

Adds an `--isolated` flag to force this behavior (ignoring an existing
virtual environment).

crates/uv-interpreter/src/python_environment.rs

@@ -126,6 +126,11 @@ impl PythonEnvironment {
             )
         }
     }
+
+    /// Return the [`Interpreter`] for this virtual environment.
+    pub fn into_interpreter(self) -> Interpreter {
+        self.interpreter
+    }
 }
 
 /// Locate the current virtual environment.