ci: Enable -Werror

Related: 14d9fa247a
Related: 5cdf428f55

.github/workflows/haskell-ci.yml

@@ -140,6 +140,9 @@ jobs:
             type:     git
             location: https://github.com/xmonad/xmonad
             branch:   master
+
+          package xmonad-contrib
+            flags: +pedantic
           EOF
           $HCPKG list --simple-output --names-only | perl -ne 'for (split /\s+/) { print "constraints: $_ installed\n" unless /^(xmonad-contrib)$/; }' >> cabal.project.local
           cat cabal.project

.github/workflows/stack.yml

@@ -99,4 +99,5 @@ jobs:
           stack test \
             --fast --no-terminal \
             --stack-yaml=${{ matrix.yaml }} \
-            --resolver=${{ matrix.resolver }} --system-ghc
+            --resolver=${{ matrix.resolver }} --system-ghc \
+            --flag=xmonad-contrib:pedantic