The prebuilt cargo (or more precisely, its libstd) cannot be used when
running cargo2android.py. Ensure that the upstream cargo binary is used.
This is automatically set when using the sandbox for Googlers. For
external contributors, raise an error if the option is not provided.
Test: As a googler, execute cargo2android.py directly; an error is
raised.
Test: As a googler, use the sandbox; cargo2android.py is executed
correctly.
Test: As an external contributor, execute cargo2android.py; an error is
raised if cargo_bin is not provided.
Bug: 217810523
Change-Id: Id42ba777b3960b13f6ef91d36eaad94df939f48e
d5d033979f