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/utils.d.ts

@@ -0,0 +1,5 @@
+/**
+ * Sanitizes an input into a string so it can be passed into issueCommand safely
+ * @param input input to sanitize into a string
+ */
+export declare function toCommandValue(input: any): string;