v2-install does not apply flags to build

**Describe the bug**

`v2-install` does not apply flags (`-fXXX`) to the build.

**To Reproduce**
Steps to reproduce the behavior:

```shell
$ cabal v2-install -fwith_xft xmobar
...
$ ~/.cabal/bin/xmobar
Warning: Xmobar must be built with the with_xft flag to support font 'xft:Oxygen Mono:size=10,DejaVu Sans Mono:size=10.' Falling back on default.
```

My xmobar config is: https://github.com/Rufflewind/config/blob/0e071d057236e7e839a44c86ffed6068b28927de/home/.config/xmobar/xmobarrc

**Expected behavior**

`xmobar` should be built `with_xft` enabled, so the warning isn't supposed to appear.

**System information**
- Operating system: `Linux 5.4.6-arch3-1 #1 SMP PREEMPT Tue, 24 Dec 2019 04:36:53 +0000 x86_64 GNU/Linux`
- cabal-install version 2.4.0.0, compiled using version 2.4.0.1 of the Cabal library
- ghc 8.6.5