Fix python import tests (#38928)

Running `spack test run <python package>` resulted in the error
```
'str' object is not callable
```
because the python executable was not set correctly.

lib/spack/spack/build_systems/python.py

@@ -173,7 +173,7 @@ def test_imports(self):
 
         # Make sure we are importing the installed modules,
         # not the ones in the source directory
-        python = inspect.getmodule(self).python.path
+        python = inspect.getmodule(self).python
         for module in self.import_modules:
             with test_part(
                 self,