revert fix secrets stored in JSON format (#473)

[fairclothjm]

This change will revert to the behavior vault-action has exhibited since v2.1.2. This ensure that all JSON is valid when set as an environment variable and that step output will be a JSON object.

I can't find the source code of github actions to actually verify this, but it seems to me that when the step output is passed to another action using the with keyword, the github action framework actually JSON.stringify's the data. This means that anything that accepts a Vault secret as input will have a valid JSON string that can then be JSON.parse'd. For example, this allows the following to work:

- name: Authenticate to Google Cloud
        uses: google-github-actions/[email protected]
        with:
          credentials_json: ${{ steps.vault_secrets.outputs.google_credentials }}