Build APK from a forked repo

[drew2a]

This PR splits build.yml into two yml files:

• build.yml: triggers by pull_request event. It has no access to the secrets.
• apk.yml: triggers by pull_request_target event. It has access to the secrets but this access is protected by rules from the apk_build environment.

Refs:

• https://dev.to/petrsvihlik/using-environment-protection-rules-to-secure-secrets-when-building-external-forks-with-pullrequesttarget-hci
• https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows
• https://docs.github.com/en/actions/deployment/targeting-different-environments/using-environments-for-deployment#environment-protection-rules