Command parsing error when using heroku run with exit code flag -x

**This project is for the Heroku CLI only. Use https://help.heroku.com for anything that is not specific to the CLI itself.**

Do you want to request a *feature* or report a *bug*?
-----------------------------------------------------
Bug report

What is the current behavior?
-----------------------------
When I run `heroku run` with the `-x` flag, it gives a `command parsing error` irrespective of the command that is used or the app that it is being run on.

Example from my terminal:
```
❯ heroku run -x echo hello world! -a test-app
Running echo hello world! on ⬢ test-app... up, run.5122 (Hobby)
command parsing error%
```

What is the expected behavior?
------------------------------
It should return a zero exit code for the above command, and return the exit code of the specified command when I'm running arbitrary commands.

Heroku version:
```
❯ heroku --version
heroku/7.42.11 darwin-x64 node-v12.16.2
```

OS version:
`Mac OSX 10.15.5`

No HTTP proxies or firewalls.