Fix snakemake environment

[tgilon]

Changes proposed in this Pull Request

This PR proposes to align the snakemake version and the associated mock_snakemake to the one used in PyPSA-Eur. This is necessary to avoid two errors. The first one is reported here PyPSA/pypsa-eur#1439 and solved here PyPSA/pypsa-eur#1440 (related snakemake bug snakemake/snakemake#3202).

The second is error is thrown with snakemake=8.27.1 and the current `mock_snakemake. Here is the traceback:

Traceback (most recent call last):
  File "technology-data/scripts/compile_cost_assumptions.py", line 3348, in <module>
    snakemake = mock_snakemake("compile_cost_assumptions")
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "technology-data/scripts/_helpers.py", line 114, in mock_snakemake
    for p in sm.SNAKEFILE_CHOICES:
             ^^^^^^^^^^^^^^^^^^^^
AttributeError: module 'snakemake' has no attribute 'SNAKEFILE_CHOICES'

I tested the current suggestion using both the snakemake rule and through terminal.

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Data source for new technologies is clearly stated.
• Newly introduced dependencies are added to environment.yaml (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the GPLv3 license.

[tgilon]

@finozzifa FYI