CI: Add macOS 1.9 job

Author: jpsamaroo

.buildkite/pipeline.yml

@@ -69,6 +69,20 @@ steps:
           julia_args: "--threads=1"
       - JuliaCI/julia-coverage#v1:
           codecov: true
+  - label: Julia 1.9 (macOS)
+    timeout_in_minutes: 90
+    <<: *test
+    agents:
+      queue: "juliaecosystem"
+      os: macos
+      arch: x86_64
+    plugins:
+      - JuliaCI/julia#v1:
+          version: "1.9"
+      - JuliaCI/julia-test#v1:
+          julia_args: "--threads=1"
+      - JuliaCI/julia-coverage#v1:
+          codecov: true
   - label: Julia 1.8 - TimespanLogging
     timeout_in_minutes: 20
     <<: *test