Fix generation of documentation. (#19)

Author: jonathanknowles

.github/workflows/build.yml

@@ -35,7 +35,15 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v3
 
-      - name: Environment
+      - name: Environment (Purge)
+        # The version of `ghc` that comes pre-installed on runner images does
+        # not have documentation for core libraries. This makes it impossible
+        # to build documentation with working hyperlinks to those libraries. To
+        # solve this problem, we purge the existing installation, which enables
+        # the next build step to install ghc from scratch, with documentation.
+        run: ghcup nuke || true
+
+      - name: Environment (Installation)
         uses: haskell/actions/setup@v2
         id: setup-haskell-cabal
         with: