Fixes#72
If an Actions job is long enough, more than an hour can pass between
creating and revoking the App token in the post-job clean up step. Since
the token itself is used to authenticate with the revoke API, an expired
token will fail to be revoked.
This PR saves the token expiration in the actions state and uses that in
the post step to determine if the token can be revoked. I've also added
error handling to the revoke token API call, as it's unlikely that users
would want their job to fail if the token can't be revoked.
With #13, this action is explicitly masking the token so we don't need to mention that we're relying on a specific format of GitHub tokens being masked by the runner.
