Fix h2 deriv import limit

[lindnemi]

If imports of h2 derivatives are e.g. limit to <10 TWh, it was possible that e.g. 8 TWh oil and 8 TWh meoh were imported and to compensate for the surplus, another 6 GWh of gas were exported. This is fixed now

Additionally a small bug in the exporter got fixed

Before asking for a review for this PR make sure to complete the following checklist:

• Workflow with target rule ariadne_all completes without errors
• The logic of export_ariadne_variables has been adapted to the changes
• One or several figures that validate the changes in the PR have been posted as a comment
• A brief description of the changes has been added to Changelog.md
• The latest main has been merged into the PR
• The config has a new prefix of the format YYYYMMDDdescriptive_title

[lindnemi]

After this PR:

(^In 2025 this is only methanol)

Before this PR: Biomass liquids was at -10 TWh and Biomass gases at +5TWh

[toniseibold]

works - would only change changelog message.