Add shebang to build script

This enables running it via simply `./Build.hs`.

It's difficult (possibly not standardized at all, I can't remember) to specify arguments here, so for now this will only work if GHC 8.10.7 is the version enabled globally. And using `cabal install --lib` (or `cabal-env`, or my script for extracting environment files from cabal scripts) is still preferable due to the slowness of `cabal run`, and HLS support. Cabal scripts are set to become more usable in 3.8: haskell/cabal#7851.

Build.hs

@@ -1,3 +1,5 @@
+#!/usr/bin/env cabal
+
 {-# LANGUAGE BlockArguments #-}
 {-# LANGUAGE ImportQualifiedPost #-}
 {-# LANGUAGE LambdaCase #-}