Initial stab at buildkite CI config

.buildkite/pipeline.yml

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+steps:
+  - label: ":runner: Launch matrix of jobs"
+    commands: |
+      AGENT_OS_AND_ARCH=( linux-x86_64 windows-x86_64 macos-aarch64 )
+      JULIA_VERSIONS=( 1.6 1.8 1.9 nightly )
+      JULIA_TEST_COVERAGE=false
+
+      for AGENT_OS_AND_ARCH in "$${AGENT_OS_AND_ARCH[@]}"; do
+        IFS=- read AGENT_OS AGENT_ARCH <<<"$${AGENT_OS_AND_ARCH}"
+        for JULIA_VERSION in "$${JULIA_VERSIONS[@]}"; do
+          # Skip julia versions that do not exist
+          if [[ "${AGENT_OS_AND_ARCH}" == "macos-aarch64" ]] && [[ "${JULIA_VERSION}" == "1.6" ]]; then
+            continue
+          fi
+
+          # Template `runtests.yml` with the values from our matrix
+          export AGENT_OS AGENT_ARCH JULIA_VERSION JULIA_TEST_COVERAGE
+          buildkite-agent pipeline upload .buildkite/runtests.yml
+
+          # For 1.9, we need a `coverage=false` build due to pkgimages
+          if [[ "$${JULIA_VERSION}" == "1.9" ]]; then
+            JULIA_TEST_COVERAGE=true buildkite-agent pipeline upload .buildkite/runtests.yml
+          fi
+        done
+      done
+    agents:
+      queue: "juliaecosystem"

.buildkite/runtests.yml

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+steps:
+  - label: ":test_tube: :${AGENT_OS}: Julia ${JULIA_VERSION}, ${AGENT_OS} ${AGENT_ARCH}, coverage=${JULIA_TEST_COVERAGE}"
+    plugins:
+      - JuliaCI/julia#v1:
+          version: "${JULIA_VERSION}"
+      # This buildkite plugin has a `command` hook inside of it,
+      # so we don't need to specify a `commands` block for this step.
+      - JuliaCI/julia-test#v1:
+          coverage: "${JULIA_TEST_COVERAGE}"
+    agents:
+      queue: "juliaecosystem"
+      os: "${AGENT_OS}"
+      arch: "${AGENT_ARCH}"