CI: fix issues after enabling return value conversion diagnostics

[camshaft]

Problem:

In #3798 I tried to enable return value diagnostics. Unfortunately, there's a few issues that came up and I probably won't have time to address them.

• Fuzzing is broken; it doesn't seem to be picking up my feature probe settings
• CBMC segfaults on several proofs; we should probably open an issue upstream even if we work around it on our end
• Some PQ code is complaining about conversion; not totally sure what's happening there.

Solution:

Go through each failure in the CI, address it, and get the PR merged