Nupkg path in RELEASES file gets updated incorrectly

I noticed something weird when I started producing deltas for releases using the update service as the `remoteReleases` URI in electron-winstaller. It seems that the update service is [returning this](https://update.electronjs.org/atom/atom-nightly-releases/win32/1.29.0-nightly5/RELEASES) for the latest release's RELEASES file now:

```
9A410F776059D4CEA85D008B461D90C7B95033EB https://github.com/atom/atom-nightly-releases/releases/download/1.29.0-nightly6/https://github.com/atom/atom-nightly-releases/releases/download/1.29.0-nightly5/atom-1.29.0-nightly5-full.nupkg 149089622
B6E8F9C8D6BE42E7888EE35CF90C4C4C10C0DB28 atom-1.29.0-nightly6-delta.nupkg 48698787
B503279BD722E84E74F611AFBF4E3F2DE60F48CD atom-1.29.0-nightly6-full.nupkg 149087747
```

On the first line, the nupkg path is double-prefixed with a URL path. The [original RELEASES file](https://github.com/atom/atom-nightly-releases/releases/download/1.29.0-nightly6/RELEASES) from my repo looks like this:

```
9A410F776059D4CEA85D008B461D90C7B95033EB https://github.com/atom/atom-nightly-releases/releases/download/1.29.0-nightly5/atom-1.29.0-nightly5-full.nupkg 149089622
B6E8F9C8D6BE42E7888EE35CF90C4C4C10C0DB28 atom-1.29.0-nightly6-delta.nupkg 48698787
B503279BD722E84E74F611AFBF4E3F2DE60F48CD atom-1.29.0-nightly6-full.nupkg 149087747
```

This could be the consequence of using a different repository for delivering releases, perhaps Squirrel is doing something sneaky there?