rust: Fix GRPC generator invocation

When using the grpc generator, the plugin expects to also have access to
the protobuf generator. This patch ensures that aprotoc will use the
Soong copy of the protobuf generator, rather than trying to use a
locally installed version from PATH.

Test: m nothing
Change-Id: I26da4d18c97017da7d8cd9515a07a2b7b2575342