Panics when used on GitHub Enterprise Server

So, I think this is a very niche use-case but it seems like `cargo-dist` doesn't recognize anything other than GitHub.com as valid host.

```
WARN skipping npm installer: couldn't compute a URL to download artifacts from
WARN skipping Homebrew formula: couldn't compute a URL to download artifacts from
× cargo dist panicked.
├─▶ at cargo-dist/src/backend/ci/github.rs:107:14
╰─▶ should not be possible to have the Github CI backend without hosting!?
```

I tried running `init` with github CI and releases enabled and it panics and I was wondering if we can relax down the host check.

Start in cargo-dist/src/backend/ci/github.rs at line 107 and reproduce init with GitHub CI and releases enabled against GitHub Enterprise Server. Trace the host check and the URL computation behind the npm and Homebrew warnings. Done means this configuration no longer panics and the affected installer artifact URLs can be computed.

Written by the indexing model from the issue text.