vk_swiftshader_icd.json is located under build/Linux. If an out/
directory exists, it was assumed to contain the file instead. Verify the
file exists under build/, as well as the ICD's shared library.

Change-Id: Iea60dcea6d247d8eb7d984dedae4f4079976fc98
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/40649Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>