Ensure Constraint of Solar and Solar-hsat with Post Discretization

[toniseibold]

Bugfix

Changes proposed in this Pull Request

When the post discretization is enabled, the function add_solar_potential_constraints is called multiple times.
The first time, the optimal capacity of generators with carrier solar-hsat is zero. However, the second time, the optimal capacity can be greater than zero which would then overly reduce the potential.
Since the p_nom is set in the next planning horizon in the rule add_brownfield switching from p_nom_opt to p_nom is the correct way to formulate the boundary condition.

Checklist

• I tested my contribution locally and it works as intended.
• Code and workflow changes are sufficiently documented.
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• Changed dependencies are added to envs/environment.yaml.
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• Changes in configuration options are added in config/config.default.yaml.
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• Changes in configuration options are documented in doc/configtables/*.csv.
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• Sources of newly added data are documented in doc/data_sources.rst.
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• A release note doc/release_notes.rst is added.
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