distribute prebuilds of @agoric/xsnap

[turadg]

What is the Problem Being Solved?

#8360 lets develops install SDK from npm instead of requiring a git checkout. A consequence of this is that @agoric/xsnap rebuilds every time running agoric install. (At least in Yarn 1, which isn't smart enough to notice when native deps don't need rebuidling.)

xsnap takes over a minute to build on my MBP.

Description of the Design

In publishing @agoric/xnsap prebuild for multiple architectures and include in the published package such that installing it from NPM brings it along.

Security Considerations

Trusting the prebuild when installing the NPM package. Validators will still install from a local build.

Scaling Considerations

n/a

Test Plan

Install on multiple OS/architectures developers use. E.g. MacOS with arm64.

Upgrade Considerations

n/a