feat: add armhf (linux-arm) self-hosted runner support [ISD-5856]

[florentianayuwono]

Problem

The canonical GitHub Actions runner fork only builds for linux-x64, linux-arm64, linux-ppc64le, and linux-s390x. There is no linux-arm (ARMv7 32-bit / armhf) build, so armhf self-hosted runners cannot be provisioned from this fork.

Solution

Add linux-arm as a supported build target. The upstream actions/runner already has most arm32 infrastructure in place (architecture enum, build defines, dev.sh mappings). The only gap is Node.js 22 for armv7l — Node.js 24 dropped armv7l support, so arm runners need Node.js 22 LTS (supported until April 2027) alongside Node.js 20.

Changes

New patch files:

• patches/runner-sdk8-arm.patch — adds NODE22_VERSION="22.22.3" and downloads the Node.js 22 armv7l binary in the existing linux-arm block of externals.sh. Verified to apply cleanly against upstream v2.334.0.
• patches/runner-main-sdk8-arm.patch — same diff targeting upstream main HEAD (c6a124e1). Verified to apply cleanly.

.github/workflows/release.yml:

• Add linux-arm to the build matrix with runs_on: arm and patch_tag: arm
• Add linux-arm-sha as a job output
• Include linux-arm in the Apply patches step condition
• Download runner-packages-linux-arm artifact in the release job
• Replace <LINUX_ARM_SHA> in the release note
• Validate SHA256 for the linux-arm package
• Upload actions-runner-linux-arm-<version>.tar.gz as a release asset

.github/workflows/patch_update_release.yml and patch_update_main.yml:

• Add arm to the for arch in ppc64le s390x loop

Notes

• Self-updater is not disabled — GitHub publishes official arm32 packages (unlike ppc64le/s390x)
• RISC-V (riscv64) is out of scope per ISD281 spec
• Spec: ISD281 (approved 22 May 2026)

Test plan

• Trigger the release workflow on an arm self-hosted runner — verify actions-runner-linux-arm-<version>.tar.gz is produced
• Confirm Node.js 20 and Node.js 22 armv7l binaries are present in _layout/externals/node20 and _layout/externals/node22
• Verify patch auto-update workflows process arm on next upstream release