make it work on CI which doesn't enter the dev shell, too

flake.nix

@@ -40,6 +40,22 @@
       checks = hslib.checks {};
       devShells = hslib.devShells {
         shellHook = ''
+          nix run .#generate-cabal-project
+        '';
+        extraBuildInputs = pkgs:
+          with pkgs; [
+            zlib
+          ];
+      };
+      packages = hslib.packages {};
+      apps.hlint = hslib.apps.hlint {};
+
+      apps.generate-cabal-project = {
+        type = "app";
+        program = "${
+          pkgs.writeShellApplication {
+            name = "generate-cabal-project";
+            text = ''
               cat >cabal.project.local <<EOF
               -- This file is auto-generated by the flake devShell's shellHook.  Do
               -- not edit this file.  Make changes in flake.nix.
@@ -51,13 +67,9 @@
                 ${tahoe-chk}
               EOF
             '';
-        extraBuildInputs = pkgs:
-          with pkgs; [
-            zlib
-          ];
+          }
+        }/bin/generate-cabal-project";
       };
-      packages = hslib.packages {};
-      apps.hlint = hslib.apps.hlint {};
 
       # Using the working directory of `nix run`, do a build with cabal and
       # then run the test suite.
@@ -69,6 +81,7 @@
             runtimeInputs = with pkgs; [pkg-config haskell.compiler.${ghcVersion} cabal-install];
 
             text = ''
+              nix run .#generate-cabal-project
               cabal update hackage.haskell.org
               cabal build all
               cabal run tests