Add hspec-discover to build-tool-depends
- Build of the cardano-explorer-api test-suite was failing due to GHC not being able to discover "hspec-discover". Adding "hspec-discover" to the build-tool-depends field of the cabal file fixes this problem. haskell.nix requires us to be a bit more explicit about this kind of thing. See https://github.com/input-output-hk/haskell.nix/issues/398 for a previous example of this issue and https://cabal.readthedocs.io/en/3.4/cabal-package.html?highlight=build-tool-depends#pkg-field-build-tool-depends for a description of the build-tool-depends field.