actions/core needs to be updated to require 1.2.6

[jontreynes]

In https://github.com/microsoft/pipelines-appservice-lib/blob/93ad0ec841bd909e0e834118ee36c3e0f2737071/packages/webclient/package.json, the dependencies for actions/core is "@actions/core": "^1.1.3",

This needs to be updated to 1.2.6 per

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