rustpkg install always installs even if it's already installed

[metajack]

It correctly detects it doesn't need to build, but it still does the cp as far as I can tell.

cc @catamorphism

[catamorphism]

I think my WIP for #9441 will fix this, but I'll be sure to test.

[catamorphism]

This appears to be fixed. I added a test case and am just running tests before submitting a pull request.