Extensions scripts not included when building with _checkouts

[PaulVMo]

When building miner (./rebar3 as validator) with blockchain as a dependency in _checkouts, the scripts in blockchain's scripts/extensions are not copied into bin/extensions in the miner directory. This means that the miner commands are not available (ledger, peer, repair, trace, etc.).

I cannot tell if this is a rebar configuration issue in the miner repo or if it is a RELX issue/limitation. However, something i noticed when trying to build images with a local custom blockchain-core