How to prep scratch org with currencies

Yes, you can insert currencies and change the corporate currency via the API. A quick way of doing this is sfdx force:data:tree:import -f CurrencyTypes.json where CurrencyTypes.json contains the following:

{
  "records": [
    {
      "attributes": {
        "type": "CurrencyType",
        "referenceId": "ref1"
      },
      "IsoCode": "INR",
      "DecimalPlaces": 2,
      "ConversionRate": 1.5,
      "IsActive":true,
      "IsCorporate": true
    }
  ]
}

Importing this into my scratch org, which was created with a default currency of GBP, (my scratch definition has a country of "GB"), resulted in my corporate currency becoming INR, and GBP being updated with an appropriate conversion rate to the corporate currency.