remote builds

[turadg]

closes: #85

Building all the use images in #69 takes much more disk space. The hacks to make that work broke the image merging job that had worked previously.

After exploring many options, this PR adopts Depot.dev to build remotely. Their concurrent multiplatform bake greatly simplifies the CI. It removes the need to self-host our ARM runner and the large persistent build cache greatly speeds up our builds (e.g. a JS change builds in minutes to resolve the image caches instead of over an hour to re-run all the chain operations.)

There are some other changes that should be evident from the commit messages.

Test plan

• Green with manually triggered run (https://github.com/Agoric/agoric-3-proposals/actions/runs/7763083187)
• yarn test works locally and includes unpassed proposals

[michaelfig]

Please mention me again when the tests are passing. Thanks!

[turadg]

Please mention me again when the tests are passing. Thanks!

@michaelfig please provide your perspective on:

• "build" command only for downstream repos
• reliance on "depot" and how it's factored
• documentation requirements

I don't expect any of those to change in the course of getting the tests back to green. (They were green when I first requested review)

Happy to sync if more context is needed.

[michaelfig]

• "build" command only for downstream repos

That looks good.

• reliance on "depot" and how it's factored

Also good. The CI changes look like I would have hoped.

• documentation requirements

I think things are sufficiently documented.

Nothing here looks like it needs any drastic changes, but I haven't yet tried to reproduce the build steps on my own machine, which I believe is a prerequisite for being able to give explicit approval.

[michaelfig]

Onward and upward!

[mhofman]

This rename causes agoric-sdk a3p-integration tests to fail.

[mhofman]

Sorry for not reviewing this earlier, and I'm not even sure I would have caught it, but synthetic-chain and base image changes cannot be coupled as there may be existing consumers of a specific version of the synthetic-chain package that use the latest a3p images also produced by this repo. In this case agoric-sdk.