Run doctest via cabal-install instead of cabal-exec

Author: eddiejessup

.github/workflows/build.yml

@@ -69,7 +69,12 @@ jobs:
       - run: cabal exec -- cabal test all --test-show-details=direct
         env:
           HSPEC_OPTIONS: --color
-      - run: cabal exec -- cabal repl --with-ghc=doctest
+      - run: cabal install exe:doctest --installdir=${{ env.RUNNER_TEMP }}
+      - run: echo "DOCTEST_NAME=doctest.exe" >> $GITHUB_ENV
+        if: runner.os == 'Windows'
+      - run: echo "DOCTEST_NAME=doctest" >> $GITHUB_ENV
+        if: runner.os != 'Windows'
+      - run: cabal repl --with-ghc=${{ env.RUNNER_TEMP }}/${{ env.DOCTEST_NAME }}
 
   success:
     needs: build