Use @actions/core.exportVariable instead of set-env (#19)

* fix: use @actions/core.exportVariable instead of set-env

`set-env` is now deprecated. `@actions/core` exposes `exportVariable`
to be used instead.

https://github.com/actions/toolkit/tree/main/packages/core#exporting-variables

* chore: bump @actions/core to v1.2.6

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;