Variable in variable is not supported. However you can achieve this via script. To do it, please switch to menu Administration / Groovy Scripts to define a script with name say get-branch-aware-key with below content:

if (build.branch != null)
    return build.getJobSecretAuthorizationContext().getSecretValue(build.branch + "_key")
else
    return "default_key" 

Then you can use @script:get-branch-aware-key@ to get value of desired secret