Skip root when assessing prefix viability (#9823)
## Summary
In CPython, it appears that `/` is not considered as a valid path in
`search_up`:
```c
static PyObject *
getpath_dirname(PyObject *Py_UNUSED(self), PyObject *args)
{
PyObject *path;
if (!PyArg_ParseTuple(args, "U", &path)) {
return NULL;
}
Py_ssize_t end = PyUnicode_GET_LENGTH(path);
Py_ssize_t pos = PyUnicode_FindChar(path, SEP, 0, end, -1);
if (pos < 0) {
return PyUnicode_FromStringAndSize(NULL, 0);
}
return PyUnicode_Substring(path, 0, pos);
}
```
```python
def search_up(prefix, *landmarks, test=isfile):
while prefix:
if any(test(joinpath(prefix, f)) for f in landmarks):
return prefix
prefix = dirname(prefix)
```
Closes https://github.com/astral-sh/uv/issues/9818.
