this is likely closer to canonical

.gitlab-ci.yml

@@ -56,7 +56,7 @@ cabal-build-and-test:
       # This is only done in CI configuration because non-CI users probably
       # want their local cabal configuration left alone.
       cat >cabal.project.local <<EOF
-      store-dir: $CI_PROJECT_DIR/.cabal
+      store-dir: $CI_PROJECT_DIR/.cabal/store
       remote-repo-cache: $CI_PROJECT_DIR/.cabal/packages
       EOF
       nix run .#cabal-test

flake.nix

@@ -84,7 +84,7 @@
               export PKG_CONFIG_PATH=${pkgs.lib.makeSearchPath "lib/pkgconfig" [pkgs.zlib.dev]}
 
               find .
-              ghc-pkg check --package-db .cabal/ghc-8.10.7/package.db
+              ghc-pkg check --package-db .cabal/store/ghc-8.10.7/package.db
               cabal update hackage.haskell.org
               cabal run tests
             '';