jiri: cd back to original directory before running jiri binary.
jiri: cd back to original directory before running jiri binary.

Without this fix, all calls to the "jiri" script will execute the "jiri"
binary inside the Jiri Root directory.  This breaks lots of things,
including "jiri cl mail", which needs to be run from within a project
directory.

Change-Id: I9050fbef01bea425b5e7a435f834db6143abe292

vanadium/go.jiri,vanadium/go.jiri