Currently, there's no way to override the Pipes script's app version and add custom variables within the Pipes script.

Feature requests:
It will be useful to have a way to override the App version of the Pipes script (atlassian/heroku-deploy).
Additionally, it is also useful to have the ability to create custom variables that is available within the Pipes script.

E.g.:

      - pipe: atlassian/heroku-deploy:1.2.1
        variables:
          HEROKU_API_KEY: $HEROKU_API_KEY
          HEROKU_APP_NAME: $HEROKU_APP_NAME
          ZIP_FILE: app-name.tar.gz
          WAIT: "true"
          CONFIG:
            APP_VERSION: 2.0.$BITBUCKET_BUILD_NUMBER
            CUSTOM_VAR: "custom var value"

Pipes repository: https://bitbucket.org/atlassian/heroku-deploy/src/master/