Merging global node environment variables from multiple config files

### Your checklist for this issue

- [x] Jenkins version `2.223`
- [x] Plugin version `1.35`
- [x] OS `Ubuntu 18.04`

### Description

When having multiple .casc files to configure different aspects of Jenkins, if multiple files have global environment variables configuration, they are not merged, but stored as a separate envVars sections:

config1.yaml
```yaml
jenkins:
globalNodeProperties:
- envVars:
env:
- key: VARIABLE1
value: foo
```
config2.yaml
```yaml
jenkins:
globalNodeProperties:
- envVars:
env:
- key: VARIABLE2
value: bar
```
resulting config in Jenkins:
```yaml
jenkins:
globalNodeProperties:
- envVars:
env:
- key: VARIABLE1
value: foo
- envVars:
env:
- key: VARIABLE2
value: bar
```

While tests show that all variables still end up in environment, when you go to Manage Jenkins -> Configure System, it only shows contents of first "envVars" block.

Expected: all global variables end up in a single "envVars" block thus avoiding visualization problem

```yaml
jenkins:
globalNodeProperties:
- envVars:
env:
- key: VARIABLE1
value: foo
- key: VARIABLE2
value: bar
```

PS It seems that originally it is not expected to have multiple instances of the same type of global node properties descriptors.