[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-rpkg-win64-gpu.ps1

@@ -0,0 +1,21 @@
+$ErrorActionPreference = "Stop"
+
+. tests/buildkite/conftest.ps1
+
+Write-Host "--- Build XGBoost R package with CUDA"
+
+nvcc --version
+if ( $is_release_branch -eq 1 ) {
+  $arch_flag = "-DGPU_COMPUTE_VER=75"
+
+  bash tests/ci_build/build_r_pkg_with_cuda_win64.sh $Env:BUILDKITE_COMMIT
+  if ($LASTEXITCODE -ne 0) { throw "Last command failed" }
+
+  Write-Host "--- Upload R tarball"
+  Get-ChildItem . -Filter xgboost_r_gpu_win64_*.tar.gz |
+  Foreach-Object {
+    & aws s3 cp $_ s3://xgboost-nightly-builds/$Env:BUILDKITE_BRANCH/ `
+    --acl public-read --no-progress
+    if ($LASTEXITCODE -ne 0) { throw "Last command failed" }
+  }
+}