Switch from `hub` to `gh`

[gh] is a [hub] reimplementation that's much faster and is now the official
Github CLI. It appears that "hub" is [deprecated].

[gh]: https://github.com/jingweno/gh
[hub]: https://github.com/github/hub
[deprecated]: https://github.com/github/hub/issues/475
Quotes around expansion: `"$(...)"`