CI: add ghc 9.12.1 case.

Author: debug-ito

.github/workflows/haskell.yml

@@ -11,6 +11,7 @@ jobs:
         plan:
           - ghc: latest
             allow-fail: true
+          - ghc: '9.12.1'
           - ghc: '9.10.1'
             freeze: '20241121-ghc-9.10.1.freeze'
           - ghc: '9.8.1'