prepare_sector_network: fix municipal waste transport links

[p-glaum]

Closes # (if applicable).

The problem was that the municipal waste transports links had the same index as solid biomass, and they were not added. With the new index for the municipal waste transports links, one can only take the values of the biomass_transport df, because the link indices changed.

Changes proposed in this Pull Request

Checklist

• [ ] I tested my contribution locally and it seems to work fine.
• [ ] Code and workflow changes are sufficiently documented.
• [ ] Changed dependencies are added to envs/environment.yaml.
• [ ] Changes in configuration options are added in all of config.default.yaml.
• [ ] Changes in configuration options are also documented in doc/configtables/*.csv.
• [ ] A release note doc/release_notes.rst is added.

[fneum]

Thanks for fixing!