Always use base Python discovery logic for cached environments (#11254)

## Summary

This is attempting to solve the same problem surfaced in #11208 and
#11209. However, those PRs only worked for our own managed Pythons. In
Gentoo, for example, they disable the managed Pythons, which led to
failures in the test suite, because the "base Python" returned after
creating a virtual environment would differ from the "base Python" that
you get after _querying_ an existing virtual environment.

The fix here is to apply our same base Python normalization and
discovery logic, to non-standalone / non-managed Pythons. We continue to
use `sys._base_executable` for such Pythons when creating the
virtualenv, but when _caching_, we perform this second discovery step.

Closes https://github.com/astral-sh/uv/issues/11237.

crates/uv-virtualenv/src/virtualenv.rs

@@ -56,7 +56,14 @@ pub(crate) fn create(
 ) -> Result<VirtualEnvironment, Error> {
     // Determine the base Python executable; that is, the Python executable that should be
     // considered the "base" for the virtual environment.
-    let base_python = interpreter.to_base_python()?;
+    //
+    // For consistency with the standard library, rely on `sys._base_executable`, _unless_ we're
+    // using a uv-managed Python (in which case, we can do better for symlinked executables).
+    let base_python = if cfg!(unix) && interpreter.is_standalone() {
+        interpreter.find_base_python()?
+    } else {
+        interpreter.to_base_python()?
+    };
 
     debug!(
         "Using base executable for virtual environment: {}",