ci: ci: fix package index update step description, note slowness

.github/workflows/ci.yml

@@ -217,12 +217,12 @@ jobs:
     # ACTIONS
     # in modular steps for faster & more focussed failures
 
-    - name: Install GHC
+    # XXX slow, I feel this should happen less often
+    - name: Update package index
       run: |
-        $stack setup --install-ghc
+        $stack update
       if: env.do-all
 
-
     - name: Build deps of hledger-lib
       run: |
         $stack build --test --bench hledger-lib --only-dependencies