cmd/release: don't add superfluous trailing slash to zip directories
cmd/release: don't add superfluous trailing slash to zip directories

Change-Id: I2122c8fee49e6fe4bbc0977770ece607e3bbc55d
Reviewed-on: https://go-review.googlesource.com/13716
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

evandbrown/build,proppy/build,proppy/build,golang/build,evandbrown/build,golang/build,golang/build,proppy/build,evandbrown/build,proppy/build,golang/build