feat: Add argument to customise command used to run checkov

[cogwirrel]

Allow customising the command to invoke checkov.

This will allow us to make use of this construct in the Nx Plugin for AWS - we use checkov for our Terraform projects and want to use it for CDK too! :)

NB this doesn't quite solve the "possible improvement to allow Docker usage" comment as the templates would need to be moved to the mounted location, however it at least moves us one step closer too :)

I also fixed a typescript build issue I was seeing:

    src/utils.ts:15:40 - error TS2345: Argument of type 'string | undefined' is not assignable to parameter of type 'string'.
      Type 'undefined' is not assignable to type 'string'.

    15     maxBuffer: 1024 * 1024 * (parseInt(process.env['CHECKOV_MAX_BUFFER_SIZE_MB']) || 1),
                                              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[cogwirrel]

Update: need a bit more control than this so we're going to just run the checkov cli ourselves. Happy for you to either merge or close this PR :)