Update buildkite for 1.12

Author: kshyatt

.buildkite/pipeline.yml

@@ -75,7 +75,7 @@ steps:
   - label: "Julia 1.12"
     plugins:
       - JuliaCI/julia#v1:
-          version: "1.12-nightly"
+          version: "1.12"
       - JuliaCI/julia-test#v1:
       - JuliaCI/julia-coverage#v1:
           codecov: true