Fixes BUILD.gn if is_fuchsia (legacy embedder) and is_debug (#25858)
Fixes BUILD.gn if is_fuchsia (legacy embedder) and is_debug (#25858)

Current recommended build methods may not trigger this issue, but
while trying different options, building flutter for the first time, I
noticed a build error because `defines` was assigned twice.

If building with both options is ever supported, I think this
fixes the error.
aam/engine,chinmaygarde/sky_engine,chinmaygarde/flutter_engine,jason-simmons/flutter_engine,flutter/engine,jason-simmons/sky_engine,jamesr/sky_engine,jamesr/flutter_engine,flutter/engine,Hixie/sky_engine,rmacnak-google/engine,devoncarew/engine,jamesr/sky_engine,chinmaygarde/sky_engine,Hixie/sky_engine,rmacnak-google/engine,devoncarew/engine,jason-simmons/flutter_engine,devoncarew/sky_engine,jamesr/sky_engine,jason-simmons/sky_engine,jamesr/flutter_engine,jamesr/flutter_engine,jamesr/sky_engine,chinmaygarde/sky_engine,jamesr/sky_engine,chinmaygarde/sky_engine,aam/engine,jamesr/sky_engine,aam/engine,Hixie/sky_engine,jason-simmons/flutter_engine,rmacnak-google/engine,aam/engine,jamesr/flutter_engine,aam/engine,rmacnak-google/engine,devoncarew/engine,jason-simmons/flutter_engine,chinmaygarde/flutter_engine,jason-simmons/sky_engine,devoncarew/sky_engine,Hixie/sky_engine,jason-simmons/sky_engine,devoncarew/sky_engine,rmacnak-google/engine,jason-simmons/sky_engine,devoncarew/sky_engine,devoncarew/engine,jason-simmons/sky_engine,jason-simmons/flutter_engine,flutter/engine,chinmaygarde/sky_engine,Hixie/sky_engine,devoncarew/sky_engine,flutter/engine,jamesr/flutter_engine,devoncarew/engine,chinmaygarde/sky_engine,Hixie/sky_engine,chinmaygarde/flutter_engine,aam/engine,jason-simmons/flutter_engine,devoncarew/engine,jamesr/flutter_engine,rmacnak-google/engine,chinmaygarde/flutter_engine,jason-simmons/flutter_engine,devoncarew/sky_engine,jamesr/sky_engine,jamesr/flutter_engine,flutter/engine,flutter/engine,jamesr/flutter_engine,chinmaygarde/sky_engine,devoncarew/engine,Hixie/sky_engine,chinmaygarde/flutter_engine,chinmaygarde/flutter_engine,devoncarew/sky_engine,flutter/engine,flutter/engine,jason-simmons/sky_engine,aam/engine,jamesr/flutter_engine,aam/engine,jason-simmons/flutter_engine,rmacnak-google/engine,chinmaygarde/flutter_engine,Hixie/sky_engine