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

[jpmcb]

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

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 aws/aws-cdk#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