ci: pin mcpp 2026.8.26.1 (#10)

The release this repository was already verified against, from its own branch
before the merge — seven repositories green, each checked to have logged
`under review: mcpp 2026.8.26.1`. This moves the pin to the published build so
the same check runs against what a user installs.

Author: Sunrisepeak

.github/workflows/ci.yml

@@ -33,7 +33,7 @@ env:
   # A version verified to build these packages, not a measured minimum. The pin
   # exists for reproducibility rather than because an older mcpp is known to
   # fail.
-  MCPP_VERSION: 2026.8.25.2
+  MCPP_VERSION: 2026.8.26.1
   XLINGS_VERSION: v2026.8.17.2
   XLINGS_NON_INTERACTIVE: '1'