Set base executable when returning virtual environment (#11209)

## Summary

I'm not sure that this has much of an effect in practice, but currently,
when we return a virtual environment, the `sys_base_executable ` of the
parent ends up being retained as `sys_base_executable` of the created
environment. But these can be, like, subtly different? If you have a
symlink to a Python, then for the symlink, `sys_base_executable` will be
equal to `sys_executable`. But when you create a virtual environment for
that interpreter, we'll set `home` to the resolved symlink, and so
`sys_base_executable` will be the resolved symlink too, in general.
Anyway, this means that we should now have a consistent value between
(1) returning `Virtualenv` from the creation routine and (2) querying
the created interpreter.

crates/uv-python/src/interpreter.rs

@@ -94,6 +94,7 @@ impl Interpreter {
     pub fn with_virtualenv(self, virtualenv: VirtualEnvironment) -> Self {
         Self {
             scheme: virtualenv.scheme,
+            sys_base_executable: Some(virtualenv.base_executable),
             sys_executable: virtualenv.executable,
             sys_prefix: virtualenv.root,
             target: None,