[WIP] ci: Add github build

[phlax]

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[phlax]

cc @nnewton @kindlehl

this is as far as i got wrt fixing envoyproxy/envoy#39287

i need to go through ci to figure out exactly how far that was

i think the general idea i had - 2 gcp/s dirs

• build cache - writeable on main, public readable for prs
• publishing - separate GCS bucket that is also public readable

i think we can make it so netlify just proxies the publishing bucket

[phlax]

had a quick look at gcs fuse - i dont see any existing github actions - so not immediately clear if it would work out of the box

i think its a really good idea tho, so im going to experiment with it - we use gsutil syncing elsewhere - so this could be really handy - will look adding an action in the envoy toolshed repo as a next step

[phlax]

not immediately clear if it would work out of the box

initial experimentation is looking very promising