Add check-latest input

In scope of this release we add the check-latest input. If check-latest is set to true, the action first checks if the cached version is the latest one. If the locally cached version is not the most up-to-date, a Go version will then be downloaded from go-versions repository. By default check-latest is set to false.
Example of usage:

steps:
  - uses: actions/checkout@v2
  - uses: actions/setup-go@v2
    with:
      go-version: '1.16'
      check-latest: true
  - run: go version

Moreover, we updated @actions/core from 1.2.6 to 1.6.0