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 <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
Reviewed-by: Michael Matloob <[email protected]>

Author: thanm

src/cmd/go/testdata/script/build_plugin_reproducible.txt

@@ -2,11 +2,6 @@
 [short] skip
 [!cgo] skip '-buildmode=plugin requires external linking'
 
-# This test has problems when run on darwin, when using a more
-# contemporary Xcode version that is unfriendly to reproducible builds
-# (see issue #64947 for the gory details).
-[GOOS:darwin] skip
-
 go build -trimpath -buildvcs=false -buildmode=plugin -o a.so main.go
 go build -trimpath -buildvcs=false -buildmode=plugin -o b.so main.go
 cmp -q a.so b.so