multichain e2e: build agoric-sdk from source in ci

[0xpatrickdev]

What is the Problem Being Solved?

Currently, starship is configured to run with agoric-sdk:dev (the latest published image) in CI:
https://github.com/Agoric/agoric-sdk/blob/master/multichain-testing/config.yaml#L4

This approach works when we need to test example contract changes, but potentially falls short if there are changes in agoric-sdk we need to test.

Description of the Design

1. Use the build directive to reference a commit hash and build the chain image from scratch:
   https://docs.cosmology.zone/starship/config/chains#build-optional

2. Make this change in a new config.ci.yaml file that's only run during CI. Preserve the existing config.yaml so other consumer do not need to build the chain every time they wish to run starship.

Security Considerations

Scaling Considerations

Test Plan

Upgrade Considerations

[0xpatrickdev]

This will likely be superseded by #10119 - we can depend on the docker image built in the PR instead