ci: Drop GHC installation via apt

The apt repo hosting these packages has been unmaintained for quite a
while, and we've excluded ~/.stack/programs from the cache long time
ago, so we can just let stack handle the installation. It's not much
slower than installing via apt, and even if it was, some matrix jobs
need to use the slow path anyway.

Related: 9fce3805fcf2 ("ci: Use system GHC in Stack to not waste GH Actions cache space")
Related: 7d10e470d71d ("ci: Avoid caching GHC")

.github/workflows/stack.yml

@@ -12,16 +12,11 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - resolver: lts-12
-            ghc: 8.4.4
-          - resolver: lts-14
-            ghc: 8.6.5
-          - resolver: lts-16
-            ghc: 8.8.4
-          - resolver: lts-18
-            ghc: 8.10.7
-          - resolver: lts-19
-            ghc: 9.0.2
+          - resolver: lts-12  # GHC 8.4
+          - resolver: lts-14  # GHC 8.6
+          - resolver: lts-16  # GHC 8.8
+          - resolver: lts-18  # GHC 8.10
+          - resolver: lts-19  # GHC 9.0
 
     steps:
       - name: Clone project
@@ -44,14 +39,6 @@ jobs:
             libxss-dev \
             #
 
-      - name: Install GHC
-        # use system ghc (if available) in stack, don't waste GH Actions cache space
-        continue-on-error: true
-        run: |
-          set -ex
-          sudo apt install -y ghc-${{ matrix.ghc }}
-          echo /opt/ghc/${{ matrix.ghc }}/bin >> $GITHUB_PATH
-
       - name: Refresh caches once a month
         id: cache-date
         # GHA writes caches on the first miss and then never updates them again;