[heroku-env] heroku-env susceptible to command injection

A command injection vulnerability affects all versions of package heroku-env. The injection point is located in lib/get.js which is required by index.js.

References

• https://nvd.nist.gov/vuln/detail/CVE-2020-28437
• https://security.snyk.io/vuln/SNYK-JS-HEROKUENV-1050432
• https://github.com/advisories/GHSA-jp45-65jw-94mj