GN: Allow building without /build/ folder. This will allow for ANGLE to be used in the Skia build. Bug: angleproject:3943 Change-Id: Idd1840f4445c63e696a7cdcd8b873623b8408306 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1835346 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>