Target directories should be cleaned before codegen

[adpaco-aws]

I've run into trouble a couple of times:

• When working on Upgrade to CBMC 5.69.1 (with fixes) #1811 , cargo-kani regressions failed because conflicts were found in the GotoC programs I had generated. I think this is due to cargo-kani using the --target-dir option; not cleaning the directory first makes us reuse some code generated earlier.
• When running the "unsupported features" experiment, it reported failures on half the crates because of some conflict. For this experiment, I need to run --legacy-linker (otherwise nothing is shown), and cleaning the experiments folder helped. I don't have a log file, but this one can be reproduced running the experiment first as is, and then with --legacy-linker.

But in general, whenever we are going to generate and link code, we should be cleaning the whole folder first. I believe some commands even use wildcards so it's no surprise this happens.