Fix up not used

Author: znicholls

.github/workflows/install-pypi.yaml

@@ -28,7 +28,7 @@ jobs:
         # Check both 'library' install and the 'application' (i.e. locked) install
         install-target: ["example-fgen-basic", "example-fgen-basic[locked]"]
         pip-install-flags: ["--only-binary example-fgen-basic"]
-        toolchain: "na"
+        toolchain: [ "not_used" ]
         include:
           # Source installs on all OS for a stable Python version
           # (no need to do more than this)