cmd/release: use dl.google.com to check for binary availability
cmd/release: use dl.google.com to check for binary availability

Change-Id: Ib2177a5360316356911002f7454778ca04689dff
Reviewed-on: https://go-review.googlesource.com/87435
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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