diag: pin MCPP_VERSION=0.0.103 to bisect the libxcb link regression (throwaway)

Author: Sunrisepeak

.github/workflows/validate.yml

@@ -47,7 +47,7 @@ env:
   # 0.0.94 fixed feature-gated `sources` under `mcpp test` (mcpp#218); 0.0.91
   # added standard = "c++fly" to the resolver grammar, so c++fly descriptors
   # get the lint WARN below, not a hard grammar-parse rejection.
-  MCPP_VERSION: "0.0.102"
+  MCPP_VERSION: "0.0.103"
 
 jobs:
   lint: