Bump @actions/core from 1.1.1 to 1.2.4 (actions#38)

Bumps [@actions/core](https://github.com/actions/toolkit/tree/HEAD/packages/core) from 1.1.1 to 1.2.4.
- [Release notes](https://github.com/actions/toolkit/releases)
- [Changelog](https://github.com/actions/toolkit/blob/master/packages/core/RELEASES.md)
- [Commits](https://github.com/actions/toolkit/commits/HEAD/packages/core)

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Author: dependabot[bot]