No build report UI for package candidates

Because `cabal-install` does not support package candidates explicitly, the only way for the doc builder to install a package candidate (in order to build its docs) is to do

```
cabal-install ... http://example.com/package/package-name-X.Y.Z/candidate/package-name-X.Y.Z.tar.gz
```

rather than simply

```
cabal-install package-name-X.Y.Z
```

That works ok, but when `cabal-install` is invoked like that it does not generate a build report, and hence we don't get build reports for candidates. We might fix this in the doc builder somehow, or else in `cabal-install`.