fix(longevity-large-partitions): disable nemesis during prepare

[roydahan]

Since large partition data validation was introduced and runs after every nemesis, it's impossible to run the nemesis during prepare because it won't find the data or partial data during the validation.

Testing

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PR pre-checks (self review)

• I added the relevant backport labels
• I didn't leave commented-out/debugging code

Reminders

• Add New configuration option and document them (in sdcm/sct_config.py)
• Add unit tests to cover my changes (under unit-test/ folder)
• Update the Readme/doc folder relevent to this change (if needed)

[roydahan]

@scylladb/qa-maintainers I'm sorry, but I want to run this one...