ci: Fix failing installation of GHC

Since actions/virtual-environments#3268, the GHC PPA is no longer
enabled by default. :-(

(It would be better to use the haskell/actions/setup action, but it
insists on doing `cabal update` even if all we need is to install GHC
and Stack, which adds noticeable delay:
https://github.com/haskell/actions/issues/29)

.github/workflows/stack.yml

@@ -25,6 +25,12 @@ jobs:
       - name: Clone project
         uses: actions/checkout@v2
 
+      - name: Prepare apt sources
+        run: |
+          set -ex
+          sudo add-apt-repository -y ppa:hvr/ghc
+          sudo apt update -y
+
       - name: Install C dependencies
         run: |
           set -ex