CMake: Move proto import path replacement to build time
CMake: Move proto import path replacement to build time

With this change, the generated build system can now decide when the SAPI
protos are out of date and need to be rebuilt. Previously, the protos had
to always be regenerated which meant compiling a big part of the codebase
after each CMake configure run, even if nothing else changed.

PiperOrigin-RevId: 455599389
Change-Id: If75b6e870cd37f3136681fc73a404c5678623e52

google/sandboxed-api,google/sandboxed-api,google/sandboxed-api,google/sandboxed-api