fix: do not call GCECredentials::onGCE if ADC has already checked

[bshaffer]

Adds a method GCECredentials::setIsOnGce, so that if ApplicationDefaultCredentials already checks that the library is running on GCE, we do not need to check again in GCECredentials.

if (ApplicationDefaultCredentials::onGce()) {
    $creds = new GCECredentials();
    $creds->setIsOnGce(true); // save the credentials a trip to the metadata server
}

[dwsupplee]

LGTM, just also raising up the PHPStan failure if you'd like to address that.