DEVOPS-3987 ci: consolidate all arch builds into a single cross-compile job

[rc-infra-pilot]

Summary

Replaces the two separate build jobs (build on macos-latest + build-linux-arm64 on ubuntu-24.04-arm) with a single build job on ubuntu-24.04 using Go's native cross-compilation.

How it works

Each platform target is a separate step that:

1. Downloads the platform-specific tsh binary (darwin-arm64 / linux-amd64 / linux-arm64)
2. Computes its MD5 and embeds it via -ldflags
3. Cross-compiles via GOOS/GOARCH env vars — no emulation needed

Targets

• bin/ridectl.macos — GOOS=darwin GOARCH=arm64
• bin/ridectl.linux — GOOS=linux GOARCH=amd64
• bin/ridectl.linux.arm64 — GOOS=linux GOARCH=arm64

Benefits

• Single runner instead of 2 (saves macos-latest minutes which are billed at 10x)
• No ARM runner dependency
• All 3 release zips produced in one job