Allow running integration tests wrapped under //testing/... on MacOS

[jhchabran]

Right now, t's not possible to do so, because we need to cross compile to produce linux/amd64 container images, but the test runners need to run against your host architecture. If your host isn't linux/amd64 you won't be able to run those tests.

This is caused by the fact that there is no straightforward way of telling Bazel to use a given toolchain for certain targets and another one for others, in a consistent fashion across the various binaries we produce (rust+go).

We really want those to be possible, as the ability of running those targets locally, with a single command is a significant advantage when iterating locally, especially when it comes to add new tests to those targets.