withCredentials() works with GitLabApiToken, but credentials() doesn't

### Jenkins and plugins versions report

Follow-up to #536 and #1754

The withCredentials() syntax does work indeed in pipelines thanks to this MR, but the credentials() one doesn't.

Environment

### What Operating System are you using (both controller, and any agents involved in the problem)?

Rocky Linux 9

### Reproduction steps

Sample pipeline:
```
pipeline {
agent any

environment {
GITLAB_API_TOKEN = credentials('tokentest')
}

stages {
stage('Hello') {
steps {
echo 'Hello World'
}
}
}
}
```

### Expected Results

Token injected as GITLAB_API_TOKEN

### Actual Results

Error message:
```text
ERROR: No suitable binding handler could be found for type com.dabsquared.gitlabjenkins.connection.GitLabApiTokenImpl. Supported types are StandardUsernamePasswordCredentials,FileCredentials,DockerServerCredentials,StringCredentials,SSHUserPrivateKey.
```

### Anything else?

_No response_

### Are you interested in contributing a fix?

_No response_