Use posix-compatible syntax for pre-push hook (#68421)
### What?
Use POSIX-compatible shell script syntax for the Husky `pre-push` hook.
POSIX shell sadly doesn't support arrays, so fall back to using
IFS-based string splitting (which is fine here).
### Why?
- Husky ignores the shebang, and simply invokes `sh`:
https://github.com/typicode/husky/issues/971
- On Debian-based distributions, [`sh` is provided by
`dash`](https://wiki.archlinux.org/title/Dash), which is POSIX
compatible, but does not support `bash`-specific features.
- The Husky documentation has a workaround for using bash, but doesn't
recommend it: https://typicode.github.io/husky/how-to.html#bash
I do most of my development on a Debian VM, which is how I noticed this.
### Test Plan
On Debian with `dash`, manually invoke it like so:
```
echo local_branch_name from_commit refs/heads/canary to_commit | sh -x .husky/pre-push canary git@github.com:vercel/next.js.git
```
