Add dynamic memory-safety analysis to CI

[twifkak]

In end-to-end-test.yml:

1. apt install valgrind into the container
2. run something like valgrind --exit-on-first-error=yes --error-exitcode=1 nginx -g 'daemon off;' & instead of nginx
3. add some mild stress testing, e.g. for ((x=0; x<100; x++)); do curl http://localhost:8080/certs/cert.cbor >/dev/null 2>&1 & done (and ditto for an SXG)

Optionally, research non-default args to valgrind, or other dynamic analysis tools.