ci: build demos

Author: frasertweedale

.github/workflows/haskell-ci.yml

@@ -720,6 +720,7 @@ jobs:
           echo "package notmuch" >> cabal.project
           echo "    ghc-options: -Werror=missing-methods" >> cabal.project
           cat >> cabal.project <<EOF
+          flags: +demos
           EOF
           if $HEADHACKAGE; then
           echo "allow-newer: $($HCPKG list --simple-output | sed -E 's/([a-zA-Z-]+)-[0-9.]+/*:\1,/g')" >> cabal.project

cabal.haskell-ci

@@ -0,0 +1,2 @@
+raw-project
+  flags: +demos