add ghc 9.12.1 to sdist check list

For 3.14 branch which ships with it.

(cherry picked from commit bddd03f)

Author: geekosaur

.github/workflows/check-sdist.yml

@@ -30,7 +30,7 @@ jobs:
         # release of a corresponding Cabal and friends. it can also be short since it's
         # highly unlikely that we are releasing really old branches.
         ghc:
-          ["9.10.1", "9.8.1", "9.6.1"]
+          ["9.12.1", "9.10.1", "9.8.1", "9.6.1"]
 
     steps: