Fix #4 Update @actions/core

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

node_modules/@actions/core/lib/file-command.d.ts

@@ -0,0 +1 @@
+export declare function issueCommand(command: string, message: any): void;