ci: bump actions off Node 20 before the June 16 forced-Node-24 cutover

[justrach]

Problem

Both workflows pin actions that still run on Node 20. GitHub annotated every job of run 27426667692 (v0.2.5825 release build):

Node.js 20 actions are deprecated. … Actions will be forced to run with Node.js 24 by default starting June 16th, 2026. Node.js 20 will be removed from the runner on September 16th, 2026.

Affected:

• actions/checkout@v4
• actions/upload-artifact@v4
• actions/download-artifact@v4
• (audit bench-regression.yml and pr-base-guard.yml for the same pins)

The forced Node 24 default lands in 4 days as of filing. If any of these actions misbehave under Node 24, the next release build breaks.

Expected

CI is green with Node 24 — either by bumping to the Node-24-ready major versions (actions/checkout@v5, actions/upload-artifact@v5, actions/download-artifact@v5) or by verifying the v4 pins under FORCE_JAVASCRIPT_ACTIONS_TO_NODE24=true before the cutover.

Fix

Bump the action majors in .github/workflows/*.yml, run release-binaries once via workflow_dispatch against the v0.2.5825 tag to verify the matrix still builds and uploads.

Note: CI-only change; no zig test applies.