Referencing //third_party/SPIRV-Tools directly only works when building
inside a Chromium checkout.

Bug: angleproject:4425
Change-Id: I70ace58145a93b747a67d98b6efd4af7e3cfd370
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/41628Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>