Change Android build workflow to nightly schedule

[Copilot]

Follow-up to #124: switches Android build CI from per-push/PR to nightly execution.

Changes

• Replace push and pull_request triggers with schedule cron (0 0 * * * - midnight UTC)
• Retain workflow_dispatch for manual runs

on:
  schedule:
    - cron: '0 0 * * *'
  workflow_dispatch:

Original prompt

I merged #124. Now make a new PR to make it nightly instead of per push or pull request

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[psiddh]

Super nice

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