ci: building isthmus native image takes long time

[benbellick]

For a PR's CI checks to go through, we have to build Isthmus native image. This takes ~15 min, which is quite a while for a relatively small repo.

We should investigate either:

• making this build step faster, or
• dropping the requirement for something simpler / faster

[bestbeforetoday]

The macOS native binary build is the one that takes a very long time. Maybe we could avoid building for macOS on every PR and only run the Linux variant.

We could also run the native binary build in parallel to the tests instead of running sequentially. This should make the end-to-end PR build faster but risks wasting build runner time on the native binary when the tests don't pass.