allow overriding SEMAPHORE_AGENT_STACK_CONFIG values with env vars

[cchristous]

Currently, the stack configuration can be provided either as a JSON file via SEMAPHORE_AGENT_STACK_CONFIG or as environment variables, but you can't combine the two. This changes it so that a user can provide a JSON file and then also provide environment variables. If there are duplicates, the environment variables take precedence.

For my use case, I'd like to supply most of my parameters via the JSON file, but I'd also like to pass the SEMAPHORE_AGENT_AMI environment variable, so I don't have to store my AMI ID in source code.

Testing

I set the SEMAPHORE_AGENT_AMI and SEMAPHORE_AGENT_STACK_CONFIG environment variables then ran a CDK diff and saw:

Resources
 └─ [~] ImageId (requires replacement)
     ├─ [-] ami-****
     └─ [+] ami-****

[lucaspin]

/sem-approve