Spawning from $PATH should run a version compatibility check

[teh-cmc]

We've recently added support for spawning from PATH in Rust, C and C++.
This is pretty nice but does have an implication: the Rerun Viewer executable in PATH should match the version of your SDK, otherwise you are likely in trouble.

The version mismatch will be detected by the viewer and generate a generic warning, but we can do a much better job in this specific instance and e.g. print out the correct download links.

The version of the viewer can be parsed from fork-exec'ing rerun --version.