mk: Fix doc tests for multiple targets

It only really makes sense to run tests for the build target anyway because it's
not guaranteed that you can execute other targets.

This is blocking the next snapshot
auto merge of #11139 : alexcrichton/rust/fix-snapshots, r=luqmana

It only really makes sense to run tests for the build target anyway because it's
not guaranteed that you can execute other targets.

This is blocking the next snapshot