Run valgrind on host mock example runs to catch more bugs in CI. These
tests would have caught the problem in #7464 before users did.
Add a list of some randomly picked examples to run, and add an option to
run the loop exactly once in the host mock routine, so the test will
actually exit under valgrind.
* ci: fix checking for presence of CI_GITHUB_API_KEY
CI_GITHUB_API_KEY is always set when evaluating job include/exclude conditions, and it only gets removed when the job starts. Move the check into the script.
* ci: split build jobs into separate shell scripts
