linuxPackages_testing: Don't recurseIntoAttrs

The -rc kernels are quite likely to break out-of-tree modules and thus
cause unnecessary Hydra failures.

(Note that linux_testing already has `hydraPlatforms = [];` but that
does not prevent the package from being built since it has reverse
dependencies. Arguably that could be considered undesirable and thus
fixing that could be considered the proper fix, but this should do
for now.)