Continue to enable TBB support, but change the default threader to Pool.

TBB can exhibit decreased performance in some cases, and it seems that
more refinement is needed with the number of work units, etc.

See:

  https://discourse.itk.org/t/8x-slower-registration-with-itk-elastix-python-api-vs-elastix-cli-minimal-reproducible-example/7736
  InsightSoftwareConsortium/ITKElastix#391

This setting is consistent with the libitk conda-forge build configuration.

PERF: Set the default threader to Pool

When building a module that depends on another ITK module (e.g. RTK
depends on ITKCudaCommon), the ITK build cache is downloaded and
extracted once in the parent directory. build-module-deps.sh then
symlinks the module directory to that cache (ITKPythonPackage ->
../ITKPythonPackage) and used to re-run the full download-and-extract
flow inside the module directory.

Re-extracting in the module directory was problematic for two reasons:

- Redundant: the same cache was just extracted in the parent directory
  with the same arguments, and the module directory writes through the
  symlink into that same shared tree.

- Recent GNU tar (openat2 jailification, e.g. Ubuntu tar
  1.35+dfsg-4ubuntu0.2+) fails with 'Invalid cross-device link' (EXDEV)
  when extracting members under ITKPythonPackage/ because the symlink
  escapes the extraction root.

The module dependency build also registers the module (e.g. CudaCommon)
into the shared ITK build tree (ITK_DIR) it builds against, so the
registration must land in the tree the parent build reuses afterwards.
Building against a module-local copy instead of the shared tree breaks
serial module dependencies.

Fix: when the parent already holds the extracted cache, build the module
directly against the shared tree with
dockcross-manylinux-build-module-wheels.sh, without re-extracting it.
When no cache is available (standalone use of build-module-deps.sh),
keep the full download-and-extract-and-build path.

BUG: Avoid re-extracting the ITK build cache for module dependencies