enable the 'Run workflow' button

[gelisam]

CI was scheduled to run automatically every month, so that if a new version of GHC or of one of hawk's dependencies gets released and causes hawk to no longer compile, I get notified and can quickly add an upper bound.

But GitHub automatically disables my monthly CI runs after a couple of months, so the inevitable happened: hawk no longer compiles with the latest version of everything (#275).

Let's enable the Run workflow button so that I can at least manually run CI, even if I can't run it automatically every month.

[gelisam]

Turns out the CI script itself bit-rotted:

1. haskell/actions/setup has been deprecated for a long time, hawk was very overdue to switch to haskell-actions/setup.
2. the CI task called "newest" is specifically testing that hawk still works with the latest version of everything, but we know that it doesn't, so I have tweaked it to temporarily test that hawk still works with ghc-8.10.4, the version in stack.yaml. I assume that hawks works with more recent versions of ghc than that, but I first want to fix CI, I will try to find which version of ghc breaks hawk in a separate PR.