ci: pin the build tool this package was released against

The pin was several releases behind, so what CI validated was not what
the ecosystem resolves. 2026.8.24.6 is the version this package's
current contents were released alongside — openkal 0.7.0 and the
implementations that follow it.

A version verified to build this package, not a measured minimum: the
pin exists for reproducibility rather than because an older mcpp is
known to fail.

Author: Sunrisepeak

.github/workflows/ci.yml

@@ -21,7 +21,7 @@ on:
   workflow_dispatch:
 
 env:
-  MCPP_VERSION: 2026.8.19.4
+  MCPP_VERSION: 2026.8.24.6
   XLINGS_VERSION: v2026.8.17.2
   XLINGS_NON_INTERACTIVE: '1'