hide stdout/stderr spew when running "jiri os-packages foo".
hide stdout/stderr spew when running "jiri os-packages foo".

"jiri os-packages" on linux uses dpkg to check if required packages are
installed.  These commands print to stdout, and also to stderr if the
package is not installed.  This output can be confusing, and should be
suppressed.

Change-Id: Idd995998b2839cf24ca088ad5f916e271776a447

vanadium/go.jiri,vanadium/go.jiri