build cmd's calculate-plutus-script-cost doesn't include script hash for reference scripts

[catch-21]

Internal/External
Internal if an IOHK staff member.

Area
Plutus Related to Plutus Scripts (Alonzo).

Summary
When using build cmd's --calculate-plutus-script-cost to produce an output of each plutus script's resource requirement and cost the scriptHash field is null for reference scripts. It is correctly populated for attached scrtips. See comparison of two transactions using the same script as reference and as attached:

$ cat ref_cost attached_cost
[
    {
        "executionUnits": {
            "memory": 663882,
            "steps": 205620764
        },
        "lovelaceCost": 53132,
        "scriptHash": null
    }
][
    {
        "executionUnits": {
            "memory": 570992,
            "steps": 176055294
        },
        "lovelaceCost": 45640,
        "scriptHash": "505755e49f9873b02258f1e6fdcc6c81446ab560a0e78180c600d9bd"
    }
]

Steps to reproduce
Steps to reproduce the behavior:

1. Use cli's build cmd to construct a transaction using a plutus reference script, use --calculate-plutus-script-cost flag to produce an output file
2. Read the output file

Expected behavior
scriptHash is always populated

System info (please complete the following information):
Node/CLI Version: 43393ac

[CarlosLopezDeLara]

Assess! impact to workflow