Part 1 of https://github.com/flutter/flutter/issues/145263

This PR updates the names of builds outside of `local_engine.json` to be
prefixed with the string `ci/` (or `ci\` on Windows). For better or
worse, the "name" field of a build is used to construct a path used as
the source directory of a copy operation (I think the CAS archive
step?). Because of that, changing the name of a build also requires
updating the build output directory of the ninja build.

This PR also adds tests to make sure the naming of these builds remains
consistent.