ci: Use xmonad master for haskell-ci workflow

Last released xmonad doesn't build with GHC 9.0 and haskell-ci doesn't
let us tweak the test matrix easily. Let's hope the stack.yml workflow
is sufficient to test backward compat.

Related: 2a1a18023a

.github/workflows/haskell-ci.yml

@@ -134,6 +134,10 @@ jobs:
           echo "package xmonad-contrib" >> cabal.project
           echo "    ghc-options: -Werror=missing-methods" >> cabal.project
           cat >> cabal.project <<EOF
+          source-repository-package
+            type:     git
+            location: https://github.com/xmonad/xmonad
+            branch:   master
           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

cabal.project

@@ -1,2 +1,7 @@
 packages:
   xmonad-contrib.cabal
+
+source-repository-package
+  type: git
+  location: https://github.com/xmonad/xmonad
+  branch: master