cmd/buildlet: make handleRemoveAll change permissions if needed
cmd/buildlet: make handleRemoveAll change permissions if needed

Fixes golang/go#32065

Change-Id: Ib2f6be05159e387374bc0255cb681c3147553973
Reviewed-on: https://go-review.googlesource.com/c/build/+/177457
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

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