try macos-13

This on the theory that it's only MPS specifically that doesn't work,
and MacOS 13 maybe (?) doesn't have MPS

Author: bmillwood

.github/workflows/checks.yml

@@ -125,7 +125,7 @@ jobs:
       matrix:
         runs-on:
           - ubuntu-latest
-          - macos-latest
+          - macos-13
         notebook:
           # - "Activation_Patching_in_TL_Demo"
           # - "Attribution_Patching_Demo"