The huge blaze that gutted Universal Studios near Hollywood at the weekend was started accidentally by workers laying down asphalt shingles on a set made to look like New York City, investigators.   "The fire was an accident, it was not intentional, it was not arson," Los Angeles County supervisor Zev Yaroslavsky told reporters.  "Two workers and a supervisor were applying asphalt shingles to the roof on the building fagade. They were using a blowtorch to heat the shingles," said LA Fire Department Chief Michael Freeman.  He said heated tools were left on the set used for outside shots for shows as varied as "Seinfeld" and "NYPD Blue."