Don't overwrite the set log_action when using --interactive

-ddump-json didn't work with --interactive as --interactive overwrote the log_action in terms of defaultLogAction.

Reviewers: bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #14078 (closed)

Differential Revision: https://phabricator.haskell.org/D4533