[CN-exec] Explain how to run and debug CI failures in ONBOARDING.md

[dc-mak]

If you look in the workflow file, you'll find the command: https://github.com/rems-project/cerberus/actions/runs/10599783302/workflow?pr=522#L85 : cd tests && ./run-cn-exec.sh

Before running the file, you can open it to see timeout 20 "${CHECK_SCRIPT}" "${SCRIPT_OPT}" "$file" &> /dev/null called in a function which is called in a loop. Alternatively, you can uncomment set -xv for debugging and run the file itself.

It will produce a bunch of output, including something like /home/dhruv/.opam/4.14.1/lib/cn/runtime/libexec/cn-runtime-single-file.sh -oq tests/cn/missing_resource_indirect.error.c which can be simplified down to
${OPAM_SWITCH_PREFIX}/lib/cn/runtime/libexec/cn-runtime-single-file.sh -oq tests/cn/missing_resource_indirect.error.c
or
./runtime/libcn/libexec/cn-runtime-single-file.sh -oq tests/cn/missing_resource_indirect.error.c

Originally posted by @dc-mak in #522 (comment)