Switch to preinstalled clang 15

Author: MeGaGiGaGon

.github/workflows/diff_shades.yml

@@ -45,7 +45,7 @@ jobs:
       HATCH_BUILD_HOOKS_ENABLE: "1"
       # Clang is less picky with the C code it's given than gcc (and may
       # generate faster binaries too).
-      CC: clang-18
+      CC: clang-15
     strategy:
       fail-fast: false
       matrix: