labgrid_test doesn't support TreeArtifact runfiles

We should allow src in labgrid_test to have TreeArtifact as runfiles. However, runfiles_dir doesn't handle that:

Error in symlink: symlink() with "target_file" directory param requires that "output" be declared as a directory (did you mean to use declare_directory() instead of declare_file()?

In order to fix this, we'll likely need to handle runfiles inside of an ctx.actions.run, given that's the only way we can traverse a TreeArtifact. This is actually the right way to do it (as opposed to doing it at analysis time, which is what we currently do), so we get a performance improvement as a bonus.