fix: lr_finder epoch loop restarting flag (#19818)

[clumsy]

What does this PR do?

lr_finder does not reset fit_loop.epoch_loop.restarting to False which causes the validation check to trigger validation on the first advance() call because of this check.

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📚 Documentation preview 📚: https://pytorch-lightning--19819.org.readthedocs.build/en/19819/

[clumsy]

Could you please check this fix @awaelchli , @carmocca ?