fix: python tix detection blocks reuse (#34768)

Now that the `tix` variant is conditional, it should also be detected
condititionally, otherwise the spec is invalid and cannot be used during
concretization.

var/spack/repos/builtin/packages/python/package.py

@@ -298,6 +298,7 @@ def determine_variants(cls, exes, version_str):
             variants += "~pyexpat"
 
         # Some variant names do not match module names
+        if "+tkinter" in variants:
             try:
                 python("-c", "import tkinter.tix", error=os.devnull)
                 variants += "+tix"