[CI] Migrate CI pipelines from Jenkins to BuildKite (#8142)

* [CI] Migrate CI pipelines from Jenkins to BuildKite

* Require manual approval

* Less verbose output when pulling Docker

* Remove us-east-2 from metadata.py

* Add documentation

* Add missing underscore

* Add missing punctuation

* More specific instruction

* Better paragraph structure

tests/buildkite/build-cpu-arm64.sh

@@ -0,0 +1,43 @@
+#!/bin/bash
+
+set -euo pipefail
+
+WHEEL_TAG=manylinux2014_aarch64
+
+echo "--- Build CPU code targeting ARM64"
+
+source tests/buildkite/conftest.sh
+
+command_wrapper="tests/ci_build/ci_build.sh aarch64 docker"
+
+echo "--- Build libxgboost from the source"
+$command_wrapper tests/ci_build/build_via_cmake.sh --conda-env=aarch64_test \
+  -DUSE_OPENMP=ON -DHIDE_CXX_SYMBOL=ON
+echo "--- Run Google Test"
+$command_wrapper bash -c "cd build && ctest --extra-verbose"
+
+echo "--- Build binary wheel"
+$command_wrapper bash -c \
+  "cd python-package && rm -rf dist/* && python setup.py bdist_wheel --universal"
+$command_wrapper python tests/ci_build/rename_whl.py python-package/dist/*.whl \
+  ${BUILDKITE_COMMIT} ${WHEEL_TAG}
+
+echo "--- Audit binary wheel to ensure it's compliant with manylinux2014 standard"
+$command_wrapper auditwheel repair --plat ${WHEEL_TAG} python-package/dist/*.whl
+$command_wrapper python tests/ci_build/rename_whl.py wheelhouse/*.whl \
+  ${BUILDKITE_COMMIT} ${WHEEL_TAG}
+mv -v wheelhouse/*.whl python-package/dist/
+# Make sure that libgomp.so is vendored in the wheel
+$command_wrapper bash -c \
+  "unzip -l python-package/dist/*.whl | grep libgomp  || exit -1"
+
+echo "--- Upload Python wheel"
+buildkite-agent artifact upload "python-package/dist/*.whl"
+if [[ ($is_pull_request == 0) && ($is_release_branch == 1) ]]
+then
+  aws s3 cp python-package/dist/*.whl s3://xgboost-nightly-builds/${BRANCH_NAME}/ \
+    --acl public-read --no-progress
+fi
+
+echo "--- Stash XGBoost CLI executable"
+buildkite-agent artifact upload ./xgboost