Mysql update table from another table with condition
you should join the tables, so  that all rows are joint together.
Before you start this use a select with the inner join to see if it fits.
UPDATE wp_postmeta_temp wpmt 
INNER JOIN wp_postmeta_import wpi ON wpi.post_id=wpmt.post_id AND wpmt.meta_key = '_variation_description'
   SET wpmt.meta_value= wpi.meta_value