cli: credential plugins declared in cdk.json stopped getting used

[humanzz]

Describe the bug

Hello,

I have a setup where I declare a credentials plugin in cdk.json e.g.

{
  "app": "node dist/lib/app USER=$USER",
  "output": "build/cdk.out",
  "plugin": [
    "my-creds-plugin"
  ]
}

This setup has been working well for me until a recent upgrade of cdk version where I moved from version 2.173.4 to version 2.176.0 where I started getting an error of the form Need to perform AWS calls for account <the account id>, but no credentials have been configured.

Using the --verbose option with cdk deploy, I noticed the following (in 2.176.0)

...
cdk.json: {
  "app": "node dist/lib/app USER=$USER",
  "output": "build/cdk.out",
  "plugin": [
    "my-creds-plugin"
  ]
}
...
merged settings: {
  versionReporting: true,
  assetMetadata: true,
  pathMetadata: true,
  output: 'build/cdk.out',
  app: 'node dist/lib/app USER=$USER',
  plugin: [ ],
...

emphasis on plugin: [ ] in merged settings:.

If I instead run cdk deploy with -p "my-creds-plugin", I get

merged settings: {
  versionReporting: true,
  assetMetadata: true,
  pathMetadata: true,
  output: 'build/cdk.out',
  app: 'node dist/lib/app USER=$USER',
  plugin: [ "my-creds-plugin" ],
...

Just seeing these longs, this suggests that something has changed, where `plugin` is no longer being mergable from `cdk.json` in newer versions.

### Regression Issue

- [x] Select this option if this issue appears to be a regression.

### Last Known Working CDK Version

2.173.4

### Expected Behavior

For the credentials plugin declared in `cdk.json` after the cdk version update

### Current Behavior

I get an error of the form `Need to perform AWS calls for account <the account id>, but no credentials have been configured`

### Reproduction Steps

As mentioned in **Describe the bug** section

### Possible Solution

_No response_

### Additional Information/Context

_No response_

### CDK CLI Version

2.176.0

### Framework Version

_No response_

### Node.js Version

20

### OS

macOS

### Language

TypeScript

### Language Version

_No response_

### Other information

_No response_

[mrgrain]

Same as #32814
Next release will fix it.

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