feat(ci, runners): added Ephemeral runners to workflow

[neo-jesse]

Integrate ephemeral runner support into main build workflow

Integrates ephemeral runner functionality into build-ot3-actions.yml to address disk space issues on persistent self-hosted runners.

Changes:

• Adds request-ephemeral-runner job that requests a unique ephemeral runner for each workflow run
• Updates run-build job to use the ephemeral runner label, ensuring builds run on clean, isolated runners
• Fixes cache zip path construction to avoid realpath issues
• Adds S3 cache pull/push support for ephemeral runners with 24h TTL and collision-free naming

Benefits:

• Each build runs on a fresh runner, eliminating state-related issues
• Maintains cache performance via S3-backed shared cache for ephemeral runners

[sfoster1]

let's do the python swap that we did in buildroot here too