pull down a previously-bootstrapped rust engine binary during development

[cosmicexplorer]

It would be nice to not have to recompile rust every time we pull master, especially if it's a change to the rust version or something which causes all dependent packages to be recompiled. Since we already build and upload the native engine binary in our CI bootstrap shards, it seems reasonable that we could expose that to the outside world in a way that would allow us to pull down the native engine upon any change to master, and only compile it ourself if we're trying to modify rust code.

This approach is somewhat similar to mozilla's "source" and "binary" mode when checking out the mozilla repo, see the --enable-artifact-builds option description in https://developer.mozilla.org/en-US/docs/Mozilla/Developer_guide/Build_Instructions/Simple_Firefox_build/Linux_and_MacOS_build_preparation.

[thejcannon]

Hello 4-year old issue 😄

We're closer than ever to doing this:

pants/src/rust/engine/BUILD

Line 25 in 2d9136a

shell_command(