CI: run the CentOS 7 jobs' actions on a Node 24 built for glibc 2.17

[garyo]

Summary

• GitHub removed Node 20 from its runners on 2026-09-23 (changelog) and now runs every JavaScript action on Node 24. The CentOS 7 jobs (VFX CY2021 and CY2022) have failed in actions/checkout since then, on Add metadata for OpenGL texture handle  #276 and on main: the runner's Node 24 needs glibc 2.28, and CentOS 7 has 2.17.
• Those jobs mounted an unofficial glibc 2.17 build of Node 20 over the runner's, which the runner no longer uses, and the ACTIONS_ALLOW_USE_UNSECURE_NODE_VERSION opt-out is gone.
• They now mount the unofficial glibc 2.17 build of Node v24.21.0 over the runner's Node 24 in the same way, and check its SHA-256 before unpacking it. The two environment variables for the old Node versions go, since the runner ignores them.

Test plan

• The glibc 2.17 Node 24 build runs in a centos:7 container (glibc 2.17), and the new install step downloads, verifies and unpacks it there.
• actionlint reports nothing new.
• The CentOS 7 CY2021 and CY2022 jobs pass on this PR.

Assisted-by: Claude Code / Claude Opus 5.5