cabal v2-haddock isn't respecting --ghc-options

**Describe the bug**
I am trying to run `cabal v2-haddock --ghc-options='-Wwarn'` but despite that, it is still erroring (due to my project-wide `-Werror`)

`cabal v2-build --ghc-options='-Wwarn'` successfully only warns instead of errors however.

**To Reproduce**
* Add `-Werror` to your cabal file ghc options
* Introduce a warning (e.g. unused variable)
* `cabal haddock --ghc-options='-Wwarn'` will fail

Please use version-prefixed commands (e.g. `v2-build` or `v1-build`) to avoid ambiguity.

**Expected behavior**
I would expect haddock generation to respect my ghc options. In this example, I would expect it to warn but not error.

**System information**
```
NixOS 20.03

cabal-install version 3.2.0.0
compiled using version 3.2.0.0 of the Cabal library

The Glorious Glasgow Haskell Compilation System, version 8.6.5
```