testsys-launcher: CFN parameter for instance type that is launched

It'd be great to provide a CFN parameter for the _type_ of instance that is launched in the nodegroup:

```go
testsysNodegroupInstanceType := awscdk.NewCfnParameter(stack, jsii.String("TestsysNodegroupInstanceType"), &awscdk.CfnParameterProps{
Type: jsii.String("String"),
Description: jsii.String("The x86 instance type to launch in the nodegroup"),
Default: jsii.String("m5.xlarge"),
})
```

But this is currently blocked on https://github.com/aws/aws-cdk/issues/25615

We could drop in a env variable at "synth-time" to get it in but it'd be great if there was just a single interface to drop these in

Start by reviewing the testsys-launcher nodegroup synthesis and the linked AWS CDK issue 25615. Determine whether the proposed TestsysNodegroupInstanceType parameter can be passed through the existing interface or whether the upstream blocker remains. Done means the launched nodegroup instance type is configurable through one interface, with m5.xlarge as the default.

Written by the indexing model from the issue text.