cmd/release: use Mac OS 10.11 builders for Mac releases
cmd/release: use Mac OS 10.11 builders for Mac releases

Change-Id: I0d82e62f5d294abcbc5225c9eb08aaf9422e6e1e
Reviewed-on: https://go-review.googlesource.com/28710
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build