Clear discovered interpreters when creating virtual environment (#15522)

## Summary

Closes https://github.com/astral-sh/uv/issues/15518.

crates/uv-python/src/interpreter.rs

@@ -106,6 +106,7 @@ impl Interpreter {
             sys_prefix: virtualenv.root,
             target: None,
             prefix: None,
+            site_packages: vec![],
             ..self
         }
     }