test(e2e): Add e2e coverage for kraft build

[srinivasr]

Prerequisite checklist

• Read the contribution guidelines (https://unikraft.org/docs/contributing/) regarding submitting new changes to the project;
• Tested your changes locally.

Description of changes

Ref: #810

Adds e2e tests for kraft build in test/e2e/cli/build_test.go, with a small fixture app under test/e2e/cli/fixtures/helloworld/ (Kraftfile, Makefile.uk, helloworld.c). No production code changed.

The building context runs kraft build --plat qemu --arch x86_64 once on the fixture, then checks that the kernel image exists and parses as an ELF (ET_EXEC, ELFCLASS32, EM_386), that it contains "Hello world!", and that the .config file was written. There are also --help output specs and two error cases (nonexistent project dir, invalid --rootfs-type). Five specs in total.

The fixture Kraftfile pins unikraft to ba13cc8 (unikraft 0.20.0) so the test doesn't change when the stable channel moves. A full commit SHA can't be used because kraft resolves core/unikraft through the manifests.kraftkit.sh index, which only lists 7-character gitshas and channels.

This spec compiles a unikernel, so the build specs are slower than the other cli specs and are skipped when not on linux/amd64, with a 20-minute timeout on the build step.

Testing

go test ./test/e2e/cli/ -run TestCli -count=1 -args -ginkgo.focus="kraft build" - 5 passed, 0 failed, 1 pending (pre-existing), 88 skipped.
golangci-lint run test/e2e/cli/... - 0 issues.

Note: on GCC 15+ hosts the pinned unikraft may fail to build (C23 makes the unprototyped uk_ctor_func_t call a hard error); UK_CFLAGS=-std=gnu11 works around it locally. CI on ubuntu-24.04 is unaffected.

[craciunoiuc]

hmm, not sure if this test should be added here

we already have a test like this building helloworld using kraft in the unikraft pipelines and in the catalog pipelines

To justify this, I think it makes sense to add a bit more tests related to how the flags work, not necessarily that the image produced is good