This should prevent the toolchain from being redownloaded on every run.

Bug: angleproject:2793
Change-Id: Iea00567c6670f87d1036a4b2cb2a807a835f2f0c
Reviewed-on: https://chromium-review.googlesource.com/1198262Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>