How can strings be appended to environment variables in ktor application.yaml?

Question

On https://ktor.io/docs/configuration-file.html#environment-variables merely a simple usage of an environment variable is demonstrated. What I'd like to achieve is a concatenation of an env var and a constant string:

kafka:
  streams:
    default:
      properties:
        state.dir: "$TMPDIR/it/streams/state"

However, when trying this or "${TMPDIR}/it/streams/state" the parser assumes that the whole string is a variable name...