[ty] Infer the Python version from the environment if feasible (#18057)

Co-authored-by: Alex Waygood <alex.waygood@gmail.com>

crates/ty_test/src/assertion.rs

@@ -501,7 +501,7 @@ mod tests {
         let mut db = Db::setup();
 
         let settings = ProgramSettings {
-            python_version: PythonVersionWithSource::default(),
+            python_version: Some(PythonVersionWithSource::default()),
             python_platform: PythonPlatform::default(),
             search_paths: SearchPathSettings::new(Vec::new()),
         };