Only use -Werror in validate

Reverts a change that made `-Werror` be applied when building in tree Cabal, even when just developing.
Puts `-Werror` back in the `cabal.project.validate` validation project.

Fixes #9866

cabal.project.validate

@@ -5,3 +5,5 @@ import: project-cabal/constraints.config
 
 tests: True
 write-ghc-environment-files: never
+program-options
+  ghc-options: -Werror

project-cabal/ghc-options.config

 program-options
-  ghc-options: -fno-ignore-asserts -Werror
+  ghc-options: -fno-ignore-asserts