Follow *_TOKEN naming convention

[20k-ultra]

balena-io/deploy-to-balena-action#213 was created because flowzone uses a different secret name for the api authorization key. Looking at the other authorization secrets for services, they all follow *_TOKEN convention.

This isn't a big issue since Flowzone is doing the following translation:

 with:
          balena_token: ${{ secrets.BALENA_API_KEY || secrets.BALENA_API_KEY_PUSH }}

However, for repositories that are using the deploy-to-balena-action and switch to Flowzone or for some reason use Flowzone + deploy-to-balena-action in separate workflows, they now need to add the same value to their repo or org under BALENA_API_KEY AND BALENA_TOKEN. Therefore, given what other authorization secrets use for convention, Flowzone should change to using BALENA_TOKEN as its secret reference.

The benefit is repositories that switch to Flowzone and have secret: inherit no additional secrets are needed for this part of Flowzone to work.