Reconfigure caching

Caching .stack-work seems sketchy -- I'd rather have a full build of the
local project each time.

~/.ghcup, on the other hand, is the toolchain and seems like a good
cache candidate.

.github/workflows/build.yml

@@ -13,8 +13,8 @@ jobs:
       - uses: actions/cache@v4
         with:
           path: |
+            ~/.ghcup
             ~/.stack
-            .stack-work
           key: ${{ runner.os }}-${{ hashFiles('**/*.cabal','**/stack.yaml') }}
           # If there's no match for key, fall back to these:
           restore-keys: |