[dev.fuzz] testing: only let workers run fuzz targets

[dev.fuzz] testing: only let workers run fuzz targets

Previously, ever worker would run all of the unit tests, benchmarks, and
examples. Only the single coordinator needs to do this.

Change-Id: I0dfa7f79b390b6c3220d8ea646e2d2312eee6bb1
Reviewed-on: https://go-review.googlesource.com/c/go/+/298809
Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Katie Hockman <katie@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>