cmd/release: remove the tmp and gocache dirs, validate it doesn't happen again
cmd/release: remove the tmp and gocache dirs, validate it doesn't happen again

Fixes golang/go#29906

Change-Id: I163aee41ea1ce5af3da058bd48b582257d96f60a
Reviewed-on: https://go-review.googlesource.com/c/159257
Reviewed-by: Filippo Valsorda <4f40cda291c5f9634e1affd3db44947af61f705c@golang.org>

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