Add comment about using a separate store-dir

Author: jneira

.azure/windows-cabal.yml

@@ -65,6 +65,8 @@ jobs:
   - bash: |
       source .azure/windows-cabal.bashrc
       # `hie` binary required locally for tests
+      # we have to use a separate store for install and builds
+      # to workaround https://github.com/haskell/cabal/issues/6483
       cabal --store-dir ${CABAL_STORE_DIR}i v2-install
     displayName: Install `hie`
   - bash: |