Enable logging of user CHERI exceptions during boot in CI

[bsdjhb]

To avoid spam, CHERI-RISC-V and Morello don't enable logging of user CHERI exceptions by default, but this means the existing regexes used during CI don't flag crashes during boot such as the issue fixed by 12f1b3ac3299 which was found on MIPS. This probably means ensuring the relevant sysctl is also a boot time tunable, and then setting it via -append (RISC-V) or the loader (Morello) during CI tests.