cmd/racebuild: destroy in-flight gomotes on interrupt
cmd/racebuild: destroy in-flight gomotes on interrupt

Updates golang/go#24354.

Change-Id: I8db0dd81a669b1e75fc82295292d2d57d1c1935b
Reviewed-on: https://go-review.googlesource.com/112878
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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