Add flag to allow failures

[elsirion]

Add flag to allow failures

[domenkozar]

That's a github feature to allow step failures, not sure it makes sense to re-implement it in this action.

[sandydoo]

For anyone looking this up in the future, you can use continue-on-error to let the job continue should the action fail:

- uses: cachix/cachix-action@v12
  continue-on-error: true
  with:
    name: my-cache
    authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'