perf(ci): built the unit test image in the debug build jobs instead of the unit-test jobs

[FlareCoding]

Summary

• Each unit-test job rebuilt the whole test image, and about three of its five minutes went to compiling a userland the build jobs had already compiled.
• make test-image is the build half of make test. The debug build jobs, which build in make test's default mode, run it on top of the userland they just compiled and upload the image, and the unit-test jobs boot that image with only QEMU installed.
• The image comes from the same run through the same Makefile path, so every commit still exercises the full image build, and a run drops from about 9.5 to 7 minutes.

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Note

Low Risk
CI/Makefile orchestration only; local make test behavior is preserved via the test-image dependency with no kernel or test logic changes.

Overview
CI now builds the QEMU unit-test disk image in debug build jobs and reuses it in unit-tests, instead of rebuilding userland twice per pipeline run.

The Makefile splits make test into make test-image (clean + unit-test-enabled image build) and make test, which still runs test-image then scripts/run_tests.sh. Debug matrix entries run make test-image with PACKAGES="none binutils" after kernel/userland and upload images/stellux-<arch>.img as a short-lived artifact.

The unit-tests job drops toolchain/sysroot caches and Limine/musl/libc++ setup; it only installs QEMU, downloads the prebuilt image, and invokes ./scripts/run_tests.sh directly (same timeout and log upload as before).

^{Reviewed by Cursor Bugbot for commit 274a18c. Bugbot is set up for automated code reviews on this repo. Configure here.}