[fuchsia] Adds --targets arg for build_fuchsia_artifacts.py (#19109)
[fuchsia] Adds --targets arg for build_fuchsia_artifacts.py (#19109)

The Fuchsia artifact builder didn't allow the end user to add more
packages to build for Fuchsia.  The current default is building only
the platform-specific packages; but packages like
`flutter/shell/common:shell_tests` are also defined for Fuchsia but
not compiled by default.

This change makes it less of a hassle to compile such packages.
chinmaygarde/flutter_engine,chinmaygarde/flutter_engine,devoncarew/sky_engine,chinmaygarde/flutter_engine,jamesr/sky_engine,devoncarew/engine,tvolkert/engine,jason-simmons/flutter_engine,devoncarew/engine,devoncarew/sky_engine,rmacnak-google/engine,jason-simmons/flutter_engine,chinmaygarde/sky_engine,aam/engine,jamesr/sky_engine,jamesr/flutter_engine,jason-simmons/flutter_engine,jamesr/sky_engine,jason-simmons/flutter_engine,Hixie/sky_engine,jason-simmons/sky_engine,tvolkert/engine,chinmaygarde/sky_engine,jamesr/flutter_engine,jamesr/sky_engine,Hixie/sky_engine,jamesr/flutter_engine,flutter/engine,Hixie/sky_engine,jamesr/flutter_engine,aam/engine,devoncarew/engine,jason-simmons/flutter_engine,flutter/engine,Hixie/sky_engine,jason-simmons/sky_engine,devoncarew/sky_engine,jamesr/flutter_engine,aam/engine,jamesr/flutter_engine,devoncarew/engine,jamesr/flutter_engine,flutter/engine,chinmaygarde/sky_engine,flutter/engine,jason-simmons/sky_engine,jamesr/flutter_engine,devoncarew/sky_engine,Hixie/sky_engine,jamesr/flutter_engine,devoncarew/sky_engine,chinmaygarde/sky_engine,rmacnak-google/engine,jason-simmons/sky_engine,tvolkert/engine,jason-simmons/sky_engine,tvolkert/engine,aam/engine,flutter/engine,flutter/engine,aam/engine,jason-simmons/flutter_engine,chinmaygarde/sky_engine,rmacnak-google/engine,jamesr/sky_engine,jason-simmons/flutter_engine,chinmaygarde/flutter_engine,jason-simmons/sky_engine,tvolkert/engine,tvolkert/engine,Hixie/sky_engine,devoncarew/sky_engine,aam/engine,chinmaygarde/flutter_engine,jamesr/sky_engine,devoncarew/engine,chinmaygarde/flutter_engine,Hixie/sky_engine,flutter/engine,rmacnak-google/engine,chinmaygarde/sky_engine,rmacnak-google/engine,tvolkert/engine,rmacnak-google/engine,chinmaygarde/sky_engine,aam/engine,jason-simmons/sky_engine,jamesr/sky_engine,jason-simmons/flutter_engine,devoncarew/sky_engine,devoncarew/engine,devoncarew/engine,aam/engine,Hixie/sky_engine,flutter/engine,chinmaygarde/flutter_engine,rmacnak-google/engine