Fix Connection Test in Job config when job credential is used from folder

[mmusenbr]

On the connection test from the folder, the GitLabConnection was initialized with the jobCredentialId, therefore the getClient assumes the system configured credential is the same as the job config credential and only looks in the syste creds store.

Fixes #1864

Testing done

I have build the plugin with the changes and deployed it on our Jenkins-Environment.

Before the Change:

1. Configure a global/System GitLab API Token Credential
2. Configure a GitLab Connection in the Jenkins System configuration (/manage/system)
3. Create a folder
4. Create a GitLab API Token Credential in the new folder (with different id)
5. Create a Job (Freestyle or Pipeline)
   1. Select the created GitLab Connection
   2. Use alternative credential
   3. Select the credentials created within the folder
   4. Click 'Test connection'
   5. ⚠️ Jenkins reports: 'Client error: No credentials found for credentialsId:
   6. 💡 The plugin ON 'Test connection' always uses the system credential store only

AFTER the change

1. Configure a global/System GitLab API Token Credential
2. Configure a GitLab Connection in the Jenkins System configuration (/manage/system)
3. Create a folder
4. Create a GitLab API Token Credential in the new folder (with different id)
5. Create a Job (Freestyle or Pipeline)
   1. Select the created GitLab Connection
   2. Use alternative credential
   3. Select the credentials created within the folder
   4. Click 'Test connection'
   5. ✔️ Connection check passes

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed

[mmusenbr]

Hi, is there anything I could help with, provide something additional, to get this fix in?