ci: Drop ppa:hvr/ghc

Breaks `apt update` on the ubuntu-22.04 runner, which is what
ubuntu-latest resolves to these days.

Fixes: 3977a7a4e2d4 ("ci: Drop GHC installation via apt")

.github/workflows/stack.yml

@@ -22,15 +22,10 @@ jobs:
       - name: Clone project
         uses: actions/checkout@v3
 
-      - 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
+          sudo apt update -y
           sudo apt install -y \
             libx11-dev \
             libxext-dev \