cabal-install-head broken for ghcjs

I observed that one fine moment the `clock` package stopped compiling with ghcjs emitting the following error:

```
System/Clock.hsc:44 directive let cannot be handled in cross-compilation mode
```

After scratching my head for some time trying to figure out what changed, it turned out to be a side effect of switching from cabal-2.4 to cabal-head version. Switching back to cabal-2.4 started building again. I never guessed that cabal could cause this sort of problem.

Could it be that the new cabal is invoking `hsc2hs` directly instead of using `hsc2hs-ghcjs`?