Consolidate CI workflows and extract shared script

[jserv]

Refine GitHub Actions workflows:

• Merge coding-style and static-analysis into a single lint job
• Add actions/cache for apt packages with workflow-hash keys
• Drop redundant build-essential install from unit-tests
• Add missing checkout step to build-lkl.yml build job
• Remove scripts/ from build artifact (integration-tests checks out)

Extract duplicated shell helpers into scripts/common.sh:

• die(), detect_arch(), download_file(), verify_sha256(), find_timeout_cmd()
• Make set_colors() portable (replace [[ ]] with [ ])
• Use absolute-path sourcing pattern across all scripts

Change-Id: Ib44af8ca4fe14982b45464476386efbb9afe3423

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Summary by cubic

Merged style and static analysis into a single CI lint job, added apt caching, and extracted shared shell helpers into scripts/common.sh to reduce duplication and improve portability. CI runs faster and fetch/build scripts are more reliable.

• Refactors

  • Consolidated coding-style and static-analysis into one lint job.
  • Cached apt packages via actions/cache with workflow-hash keys.
  • Extracted helpers to scripts/common.sh (die, detect_arch, download_file, verify_sha256, find_timeout_cmd) with absolute-path sourcing and POSIX-friendly set_colors().
  • Unified arch detection, downloads, and SHA256 checks across scripts; updated rootfs cache key to include scripts/common.sh.
  • Ensured unit tests avoid LKL linkage by filtering LDFLAGS.

• Bug Fixes

  • Added missing actions/checkout in the LKL build workflow.
  • Made build-lkl.sh reliably checkout branches/tags and bare commit SHAs (handles shallow clones).
  • Dropped redundant build-essential install from unit-tests; only install needed packages.
  • Removed scripts/ from build artifacts; integration job checks out sources directly.

^{Written for commit 00005f1. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 10 files