CI: split CLI and nginx checks into parallel jobs

CI: split CLI and nginx checks into parallel jobs

Previously, check-pr runs all build configurations sequentially, so
an early failure prevents later configurations from being tested.

Split CLI checks into matrix configurations and nginx checks
into separate njs, QuickJS, and QuickJS-NG matrix jobs.  Disable
fail-fast and use fresh build directories without make clean.

Preserve the nginx module and SSL configure flags, sanitizer and
debug-memory settings, QuickJS-NG version, checkout action revision,
test commands, and failed-test retries.

Make engine selection explicit: njs jobs no longer auto-detect
QuickJS-NG, and the QuickJS CLI build uses an explicit library.