I was a bit surprised to find myself writing this... is it because it's a dreadful idea?

fun Test.envVarInput(envVar: String) {
  val value = providers.environmentVariable(envVar)
  inputs.property(envVar, value).optional(true)
  doFirst {
    val dbTypeResolved = value.orNull
    if (dbTypeResolved != null) {
      environment(envVar, dbTypeResolved)
    } else {
      environment.remove(envVar)
    }
  }
}

tasks.test {
  envVarInput("ENV_VAR_READ_BY_TESTS")
}

You could use an anonymous object instead, because environment is a MapString, Object and at execution time, the value will be retrieved by calling toString, but you still need the input declaration to wire the env as input.

Cool, thanks