When using <c:select> in a jelly file and readonlyMode is enabled, the dropdown is not disables and the add button still works.

I noticed this while working on configfile provider plugin

How to reproduce

1. Install these credentials:
   credentials-binding
   extended-read-permission
   matrix.auth
2. configure a read only user that has extended read on job
3. create a user pwd credential
4. create freestyle job and add a credential binding for the user/pwd
5. View the job config with the read only user

Expected

The dropdown is disabled and only shows the active value.
The add button is disabled or not shown at all

Actual

The dropdown is empty and the button is active but nothing happens when clicking on it.

In another scenario with configfile provider, there is the ability to view a configfile from a job config. Here the user has job config permissions. On a change that I'm currently working on I use the readonlyMode setting that should disable all inputs but the credentials selection is not disabled.

So when a user has no permissions to create credentials the add button shouldn't be shown at all I think