Add ghc environment files to the CI for doctest

When running doctest outside of a nix-shell, it is failing to find
build depends.

Change-Id: I7aaa4e9f1f4b656ca76003643a98aaf04b1d07b4

.github/workflows/main.yml

@@ -28,7 +28,7 @@ jobs:
           ${{ runner.os }}-${{ matrix.ghc }}-
           ${{ runner.os }}-
     - run: cabal update
-    - run: cabal build --enable-tests --flags=ci all
+    - run: cabal build --enable-tests --flags=ci all --write-ghc-environment-files=always
     - run: cabal test --enable-tests --flags=ci --test-show-details=direct all
     - run: cabal haddock all
     - run: cabal sdist all