Tune down flux image reg rate limiting for GCP

[squaremo]

GCP has some pretty strict limits on the rate you can query its container registry. https://cloud.google.com/compute/docs/api-rate-limits (it's not totally clear to me that that applies to GCR -- I guess why wouldn't it? But at least one customer has run into rate limiting.) See fluxcd/flux#1016

However, lowering the defaults in fluxd would make it rather slow for anyone that doesn't encounter these limits. So, better to set these just for GCP, if we can.

[rade]

Just because a user is on GCP/GKE, does not mean they use GCR. In fact I suspect most don't.

[squaremo]

Yeah that's a good point. There may also be some poor souls who are using gcr from other-than-gcp (maybe??).