CI: use cabal 3.8.1.0, --ghc-options=-Werror.

Previously we had to work around a cabal-install bug that caused
`-Werror` to react to dependencies as well as local packages,
but this is fixed in cabal 3.8.1.0.  There are some cases in which
building dependencies only doesn't work (see #8084).

.github/workflows/ci.yml

@@ -47,32 +47,32 @@ jobs:
       matrix:
         versions:
           - ghc: '8.6.5'
-            cabal: '3.2'
+            cabal: 'latest'
             cabalopts: '-f-embed_data_files'
             testopts: '--test-option=--hide-successes --test-option=--ansi-tricks=false'
           - ghc: '8.8.4'
-            cabal: '3.2'
+            cabal: 'latest'
             cabalopts: '-f-lua -f-server --enable-benchmarks'
             testopts: '--test-option=--hide-successes --test-option=--ansi-tricks=false'
           - ghc: '8.10.7'
-            cabal: '3.2'
+            cabal: 'latest'
             cabalopts: ''
             testopts: '--test-option=--hide-successes --test-option=--ansi-tricks=false'
           - ghc: '9.0.2'
-            cabal: '3.4'
+            cabal: 'latest'
             cabalopts: ''
             testopts: '--test-option=--hide-successes --test-option=--ansi-tricks=false'
           - ghc: '9.2.3'
-            cabal: '3.6'
+            cabal: 'latest'
             cabalopts: ''
             testopts: '--test-option=--hide-successes --test-option=--ansi-tricks=false'
           - ghc: '9.4.2'
-            cabal: '3.8'
+            cabal: 'latest'
             cabalopts: ''
             testopts: '--test-option=--hide-successes --test-option=--ansi-tricks=false'
 
           - ghc: '9.4.2'
-            cabal: '3.8'
+            cabal: 'latest'
             cabalopts: '--allow-newer --constrain "mtl >= 2.3.1"'
             testopts: '--test-option=--hide-successes --test-option=--ansi-tricks=false'
     steps:
@@ -105,17 +105,13 @@ jobs:
           dist-newstyle
         key: ${{ runner.os }}-${{ matrix.versions.ghc }}-${{ matrix.versions.cabal }}-cabal-local-${{ secrets.CACHE_VERSION }}
 
-    - name: Install dependencies
+    - name: Update cabal
       run: |
           cabal update
-          cabal build all ${{ matrix.versions.cabalopts }} --dependencies-only --enable-tests --disable-optimization
 
     - name: Build and test
       run: |
-          cabal build ${{ matrix.versions.cabalopts }} --enable-tests --disable-optimization 2>&1 | tee build.log
-          # fail if warnings in local build
-          ! grep -q ": *[Ww]arning:\|: *error:" build.log || exit 1
-          cabal test all ${{ matrix.versions.cabalopts }} --disable-optimization ${{ matrix.versions.testopts }}
+          cabal test ${{ matrix.versions.cabalopts }} --enable-tests --disable-optimization --ghc-options=-Werror all
 
   windows: