Prevent ameba from being installed during postinstall

[jwoertink]

Ref: luckyframework/lucky#1524

Since Lucky includes Avram as a dependency, when you install Lucky, it runs Avram's postinstall script which in turn will run shards build from the avram project. This command will install the development_dependencies.

To fix that, we add an ENV var to skip installing development shards. If that ENV var exists, then we install without development dependencies. You can still run ./script/precompile_tasks locally if you need.