fix: configure coverage helpers for test exec group

[fmeum]

They are run on the test action's execution platform, which is resolved for the test exec group, not the default one.

[aignas]

LGHM, but I lack the context behind the reason why we are fixing it. I am +1 for merging but love if you could point me to extra docs.

Does this fix a particular issue or does this just make things more tidy?

[fmeum]

The docs don't exist yet, but I'm adding them in bazelbuild/bazel#25996. We can link to that once it lands (and wait for it to get merged if you prefer that).

In a cross-platform build with custom exec constraints on either the test or default exec group, using the wrong exec group would potentially result in a failure to run the coverage tool (e.g. if the test runs on Linux, but "build" actions for the py_test build on macOS - this would try to run the tool in a macOS JDK on Linux).

[rickeylev]

Would this fix #2850 ? The error in that report seems related to the test toolchain

[fmeum]

It wouldn't, but I will send a fix for that other issue.