Fixes #4436

### Problem

Currently, the inline native engine build in `generate-bintray-manifest.sh` has been failing silently on Linux. When a release build is conducted, this causes an outright failure due to a malformed `native-engine.bintray.json`.

### Solution

Drop 32-bit binary builds on Linux in favor of a singular 64-bit copy-from-cache mode gleaned from the OSX logic. Also, add a `set -e` to error the script if issues are encountered so they may be caught earlier.

### Result

Greener builds.