[Security] Update @actions/core to 1.2.6

[past-due]

See blog post: https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/

Action authors who are using the toolkit should update the @actions/core package to v1.2.6 or greater to get the updated addPath and exportVariable functions.

Action and workflow authors who are setting environment variables via STDOUT should update any usage of the set-env and add-path workflow commands to use the new environment files.

If you need to log untrusted information such as issue titles, bodies, or commit messages to STDOUT we recommend that you disable command processing prior to doing that.