[infra][ftx] Use LUCIEXE_FAKEBUILD instead of binary proto for testing Change-Id: I19cdbee64d57dbbbedadbd1d4528affb85da7261 Reviewed-on: https://fuchsia-review.googlesource.com/c/infra/infra/+/862245 Commit-Queue: Rahul Bangar <rahulbn@google.com> Reviewed-by: Vinicius Felizardo <felizardo@google.com>