CI: run cabal update before cabal build

... to hopefully fix CI.

The latest version of cabal returns this without the update first:

Run cabal build
  cabal build
  shell: /usr/bin/bash -e {0}
Error: [Cabal-7160]
The package list for 'hackage.haskell.org' does not exist. Run 'cabal update' to download it.

.github/workflows/build.yml

@@ -54,6 +54,7 @@ jobs:
  - run: make test-sdist
  #- run: make test-via-cabal
  - run: cabal configure --enable-tests --enable-benchmarks --ghc-options="-O0"
+ - run: cabal update # latest version of cabal requires a package list to run...
  - run: cabal build
  - run: cabal test
  - run: cabal haddock