Instead of using the GH CLI (which requires some local setup), we can
just use cURL and JQ. Additionally, the original logic required setting
up alias.update-cache. After the change, this configuration is no longer
necessary.
validation on mac:
```
❯ ./update-cache.sh
Local branch 'trunk-cached' successfully updated to f26974b16d (from 19
hours ago).
```
Reviewers: Ken Huang <s7133700@gmail.com>, Chia-Ping Tsai
<chia7712@gmail.com>
Uses the `gh` CLI to find the latest trunk commit which has been cached by GitHub actions. By basing PRs off of this
ref rather than HEAD, we will see fewer cache misses in our CI builds.
Reviewers: Chia-Ping Tsai <chia7712@gmail.com>, David Arthur <mumrah@gmail.com>