Each interation of saving the cache would copy the whole `cabal` store
into a subfolder in the CACHE_DIR rather than copying the contents of
the cabal store into the cache dir. This resulted in a cache which
looked like:

```
/builds/ghc/ghc/cabal-cache/cabal/cabal/cabal/cabal/cabal/cabal/cabal/cabal/cabal/cabal/
```

So it would get one layer deeper every CI run and take longer and longer
to compress.