CVE-2020-15228: Improper Input Validation

In the `@actions/core` npm module before version 1.2.6,`addPath` and `exportVariable` functions communicate with the Actions Runner over stdout by generating a string in a specific format. Workflows that log untrusted data to stdout may invoke these commands, resulting in the path or environment variables being modified without the intention of the workflow or action author. The runner will release an update that disables the `set-env` and `add-path` workflow commands in the near future. For now, users should upgrade to `@actions/core v1.2.6` or later, and replace any instance of the `set-env` or `add-path` commands in their workflows with the new Environment File Syntax. Workflows and actions using the old commands or older versions of the toolkit will start to warn, then error out during workflow execution.