feat(rustup): complete formula for rustup

## Summary

The `rustup` formula needs to be completed with proper values.

## Required Changes

1. Add a proper description
2. Set homepage URL
3. Remove all template comments
4. Set the correct license
5. Add proper `url` and `sha256` for a release
6. Update the `test` block with a real test

## References

- [Homebrew Formula Cookbook](https://docs.brew.sh/Formula-Cookbook)

Locate the rustup formula in the repository and read the Homebrew Formula Cookbook reference first. Complete the description, homepage, comments, license, release URL, checksum, and test block as listed; done means each required value is populated and the formula has a real test.

Written by the indexing model from the issue text.