External loader: don't do process IO on compute threadpool

[teh-cmc]

Not quite sure what I was thinking when i wrote this 😒

• Fixes Loading data with RAYON_NUM_THREADS=1 does nothing #4938
• Fixes Loading data with more than 1 space view deadlocks if RAYON_NUM_THREADS=2 #4939

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[emilk]

Good catch :)