Fix Node 20 deprecation

[evsok]

Fix Node 20 deprecation in github-sync CI workflows

Summary

GitHub is retiring the Node 20 action runtime; actions bundling a Node 20 (or older) entrypoint emit deprecation warnings today and will eventually stop running. This PR bumps the one affected action in this repo's CI.

Changes

Action	Before	After (node24)	Occurrences
actions/checkout	@v1	@v5	2

Files touched: .github/workflows/linter.yml, test.yml. (checkout@v1 is node12-era, so it was already emitting deprecation warnings.)

Not applicable / out of scope

• This repo's own action (action.yml) is a Docker container action (runs.using: docker), so it is not subject to the Node runtime deprecation — no change needed there.
• azohra/shell-linter@latest — Docker container action, unaffected.
• ./ (uses the local Docker action for a self-test) — unaffected.

Verification

Confirmed via the pinned action.yml (runs.using:):

• checkout@v1 → node12; @v5 → node24

Diff is version strings only, no other edits.

Runner requirement

Both jobs run on ubuntu-latest (GitHub-hosted), which already supports the Node 24 runtime — no runner action needed.

Testing

• Run the lint + test workflows and confirm the checkout step runs without Node deprecation warnings.