build: keep the CCCL submodule out of the sdist

Follow-up to #2940. That change vendored CCCL as the submodule
cuda_core/third_party/cccl and grafted its libcudacxx/include tree into the
sdist through cuda_core/MANIFEST.in.

The setuptools-scm 10 git file finder lists the files of an initialized
submodule, so building an sdist from a checkout that has the submodule
checked out now packs the entire CCCL tree (cub, thrust, docs, tests, ...)
into the sdist, although cuda.core only needs the libcu++ headers that
MANIFEST.in already adds explicitly. The same listing is also walked on every
egg_info run, which slows down wheel builds.

Mark the submodule export-ignore, which the finder honors, so it is skipped.
The MANIFEST.in graft still provides the headers and licenses.

Author: juenglin

.gitattributes

@@ -14,3 +14,11 @@ pixi.lock merge=binary linguist-language=YAML linguist-generated=true
 # `setuptools-scm` to correctly determine the version when building from a git
 # archive.  See #1609.
 .git_archival.txt export-subst
+
+# cuda.core compiles against the libcu++ headers of the vendored CCCL submodule
+# only; cuda_core/MANIFEST.in grafts those headers (and their licenses) into the
+# sdist explicitly.  setuptools-scm 10 lists the files of an initialized
+# submodule, which would otherwise add the entire CCCL tree to the sdist and
+# make every egg_info walk it.  "export-ignore" makes its file finder skip the
+# submodule.  See #2940.
+cuda_core/third_party/cccl export-ignore