devops: split build action steps into their own composite actions

[arocull]

• Splits Unity build and AWS Upload steps into their own composite actions for more organized workflows (should make it easier to add more target platforms later on)
• Provides additional context to build messages like commit name
• Rejects uploads if they're under a certain size (1000 KB)

I feel like maybe some of these should be shell scripts, but whatever

[jake-white]

neat