This is a simple adjustement for the TTL value of cached OIDC userinfo when using
bearer token authentication.

There is no need to substract 1 from computed TTLs as it can lead to cache userinfo
with a 0 seconds TTL when access token is about to expire and thus send multiple
userinfo requests to the OIDC server on successive calls to the Web API.

I noticed that behavior while testing authentication integration in swh-web-client.