scons: Add PROTOC from the environment
scons: Add PROTOC from the environment

This patch adds PROTOC to the build environment.
(transplanted from 7867c5f28b90e002fb247b65042f93ac3a6fd766)

andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin