Avoid the use use of workflow.source_path

[koen-vg]

The above function returns a path to a cached source file, which changes for every run, causing the resulting parameter to change and the solve_network rule to re-run every time. The problem was introduced in d145758

I have tested the solution, and also tested to make sure this works when pypsa-eur is used as a module.

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]

Oh, interesting! This is a big downside of using workflow.source_path() for any place (input, param) and probably not intended by the snakemake developers. Thanks for finding the issue and providing the fix!