cmd/racebuild: destroy temp instances after use
cmd/racebuild: destroy temp instances after use

Change-Id: Iea4aa0aa4977a78df08918c96bb9c7753f4cc872
Reviewed-on: https://go-review.googlesource.com/32161
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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