Stack does not respect use of system ghc when it cannot find installation keys for the OS

### General summary/comments (optional)

I am trying to build a project on Arch Linux on ARMv7 (Raspberry Pi 4), and it cannot find the OS Key for `linux-armv7-tinfo6`. This is expected behavior. However, I managed to install ghc 8.6.1 via pacman. I figured I would be able to use [this resolver](https://www.stackage.org/blog/2018/09/announce-ghc-8.6-on-nightlies) with the flag `system-ghc: true` to at least get a build started if not fully finished. This is not the case.

### Steps to reproduce

The full steps are not needed to understand the gist of this issue (I think). But for completeness:

1. Install Arch Linux on a Raspberry Pi
2. Install Stack: `pacman -S stack`
3. Upgrade to Stack 2.1.3: `stack upgrade`
4. Install GHC: `pacman -S ghc`
5. Set up trivial project that uses the resolver: `resolver: nightly-2018-09-29`
6. Set the flag `system-ghc: true`
7. Build the project

### Expected

The build should start with the system-ghc

### Actual

```
$ stack build
Unable to find installation URLs for OS key: linux-armv7-tinfo6
```

### Stack version

```
$ stack --version
Version 2.1.3, Git revision 636e3a759d51127df2b62f90772def126cdf6d1f (7735 commits) arm hpack-0.31.2
```

### Method of installation

`stack upgrade . pacman -S stack`