Interrupting litani run leaves Z3 process running and hogging CPU

[rod-chapman]

If a litani-driven proof run appears to "hang up" or seems to be failing to make progress, it can be interrupted at the command-line with a "Control-C" keypress.

Unfortulately, this almost always the underlying prover (Z3 or Bitwuzla, for example) process running, and consuming 100% CPU time.

Can the script(s) be updated so that a "Control-C" always kills the litani process and all its sub-processes please?

[tautschnig]

I believe this is another instance of awslabs/aws-build-accumulator#162 - though perhaps now it gives us a better way to reproduce. We'll need to resurrect the candidate fix at awslabs/aws-build-accumulator#170.

[rod-chapman]

Also related to: diffblue/cbmc#8485