cmd/release: clean gocache and tmp in source mode too
cmd/release: clean gocache and tmp in source mode too

I missed this in CL 159257 when fixing golang/go#29906 the first time.

Fixes golang/go#30181 (round two)

Change-Id: Iae90b5b19803e1ef7297b57bae7644e16c44efdc
Reviewed-on: https://go-review.googlesource.com/c/161967
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

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