Changed!: Change TERMUX_IS_DEBUG_BUILD env variable name to TERMUX_IS_DEBUGGABLE_BUILD and change GITHUB_DEBUG_BUILD release type to just GITHUB

This is being done since github release artifacts may be converted to non-debuggable if felt appropriate in future or at least is a more appropriate name. Signing keys can stay same as per commit/push builds. Currently, no changes are planned, just future proofing. The `TERMUX_IS_DEBUGGABLE_BUILD` env variable could be used to differentiate if needed.

Will also check if Termux app is installed and not disabled and will calculate APK signature only when needed since its a slightly expensive operation.

This commit breaks da07826a.