cmd/go: turn back on build_plugin_reproducible script test
Re-enable the build_plugin_reproducible script test now that CL 586079
(more linker changes to work around xcode problems on Darwin with
build reproducibility) is in.
Fixes #64947.
Cq-Include-Trybots: luci.golang.try:gotip-darwin-amd64-longtest
Change-Id: Ice5bc5b809fa7fee689b78fcb874049493bc2c5c
Reviewed-on: https://go-review.googlesource.com/c/go/+/585356
TryBot-Bypass: Than McIntosh <thanm@google.com> Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: Michael Matloob <matloob@golang.org>