Replace Buildkite CI with GitHub Actions self-hosted runners

Migrate GPU tests from Buildkite to GitHub Actions using self-hosted
runners with the `gpu` label (Tesla T4 on arctic1).

- Add .github/workflows/GPU.yml for GPU tests on [self-hosted, Linux, X64, gpu]
- Delete .buildkite/pipeline.yml

Part of the SciML-wide migration from Buildkite to GitHub Actions
self-hosted runners. See ChrisRackauckas/InternalJunk#16
for runner infrastructure details.

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>

Author: ChrisRackauckas

.buildkite/pipeline.yml

@@ -0,0 +1,38 @@
+name: "GPU Tests"
+
+on:
+  pull_request:
+    branches:
+      - master
+    paths-ignore:
+      - 'docs/**'
+  push:
+    branches:
+      - master
+    paths-ignore:
+      - 'docs/**'
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: ${{ github.ref_name != github.event.repository.default_branch || github.ref != 'refs/tags/v*' }}
+
+jobs:
+  gpu-tests:
+    name: "GPU Tests"
+    runs-on: [self-hosted, Linux, X64, gpu]
+    timeout-minutes: 60
+    steps:
+      - uses: actions/checkout@v6
+      - uses: julia-actions/setup-julia@v2
+        with:
+          version: "1"
+      - uses: julia-actions/julia-buildpkg@v1
+      - uses: julia-actions/julia-runtest@v1
+        env:
+          GROUP: "GPU"
+      - uses: julia-actions/julia-processcoverage@v1
+      - uses: codecov/codecov-action@v5
+        with:
+          files: lcov.info
+          token: ${{ secrets.CODECOV_TOKEN }}
+          fail_ci_if_error: false