Fix LKL nightly CI: add tools/lkl build phase

[jserv]

Nightly build produced vmlinux/lkl.o but never ran make -C tools/lkl, so liblkl.a was missing and both x86_64 and aarch64 jobs failed at verify step.

While here, consolidate the workflow:

• Collapse duplicate x86_64/aarch64 jobs into a matrix strategy
• Scope permissions per-job (contents:write only on publish-nightly)
• Add concurrency group to prevent schedule/dispatch races
• Replace full git clone with shallow fetch (depth=1)
• Use github.repository instead of hardcoded repo for fork compat
• Add timeout-minutes on build jobs
• Guard against empty upstream HEAD from transient ls-remote failures
• Shift schedule from 03:00 to 00:30 UTC (08:30 TST)

Change-Id: I9747519501a02b419fe6942237b26abe94825485

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Summary by cubic

Fixes the nightly LKL build by adding the missing make -C tools/lkl step so liblkl.a is produced and symbol checks pass on both x86_64 and aarch64. Also streamlines the workflow and reduces flakiness in scheduling and publishing.

• Bug Fixes

  • Build liblkl.a by running make -C tools/lkl after the kernel build to restore passing verifies.

• Refactors

  • Use a matrix to build x86_64 and aarch64; add timeout-minutes and a shallow fetch.
  • Scope permissions per job; use github.repository for fork compatibility.
  • Add a concurrency group to avoid schedule/dispatch races; publish via delete-then-create on lkl-nightly.
  • Fail fast if upstream HEAD cannot be resolved; shift cron to 00:30 UTC (08:30 TST).

^{Written for commit c0c8d84. Summary will update on new commits.}

[cubic-dev-ai]

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