test_no_std does not test no_std anymore

[WesleyRosenblum]

Problem:

./scripts/test_no_test was using the thumbv7m-none-eabi target as a way to confirm that the s2n-quic-core crate does not have any dependency on the Rust standard library. At some point thumbv7m-none-eabi was changed to start including rust-std so we can no longer rely on it to verify no_std compatibility

Solution:

A first attempt at trying the method in https://blog.dbrgn.ch/2019/12/24/testing-for-no-std-compatibility/ did not work