.drone.yml variable substitution not properly escaped

Drone Support
1

.drone.yml

  commands:
  # var1 is undefined
  - var2="abcd"
  - echo ${var1:-${var2}}
  # print empty line, which is intended
  - echo ${var1:-$${var2}}
  # print "$", not escaped!
  - echo ${var1:-$var2}
  # print "abcd", which is intended

${var1:-$${var2}} should pre-process to ${var2} if var1 is undefined