Fix broken equity constraints logic

[ricnogfer]

…A-Eur-Sec

Closes # (if applicable).

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.

[pz-max]

@ricnogfer can you expand your PR description?
Looking at your code, do we need the .to_xarray() part at all?
I think it worked in my initial PyPSA-Eur linopy conversion without to: #448

[ricnogfer]

For reference, please see PR #679 instead.