Bump libherokubuildpack in the rust-dependencies group

Bumps the rust-dependencies group with 1 update: [libherokubuildpack](https://github.com/heroku/libcnb.rs).


Updates `libherokubuildpack` from 0.30.3 to 0.30.4
- [Release notes](https://github.com/heroku/libcnb.rs/releases)
- [Changelog](https://github.com/heroku/libcnb.rs/blob/main/CHANGELOG.md)
- [Commits](heroku/libcnb.rs@v0.30.3...v0.30.4)

---
updated-dependencies:
- dependency-name: libherokubuildpack
  dependency-version: 0.30.4
  dependency-type: direct:production
  update-type: version-update:semver-patch
  dependency-group: rust-dependencies
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]