(fix) re-enable format option for jsonnet command

[tphoney]

➜  drone-cli git:(re-enable_format) ✗ release/linux/amd64/drone jsonnet --source=bla.jsonnet  --format --stdout=true
kind: pipeline
name: default
platform:
  arch: amd64
  os: linux
  prune: false
steps:
- image: alpine:latest
  name: test
  pull: always
  settings:
    config: policy.yml
    project: test-dev
    prune: false
    set_env_vars:
      prune: false
➜  drone-cli git:(re-enable_format) ✗ release/linux/amd64/drone jsonnet --source=bla.jsonnet  --stdout=true
{
   "kind": "pipeline",
   "name": "default",
   "platform": {
      "arch": "amd64",
      "os": "linux",
      "prune": false
   },
   "steps": [
      {
         "image": "alpine:latest",
         "name": "test",
         "pull": "always",
         "settings": {
            "config": "policy.yml",
            "project": "test-dev",
            "prune": false,
            "set_env_vars": {
               "prune": false
            }
         }
      }
   ]
}