Add 1.11 CI (#306)

Also disable nightly until it's on a different LLVM version as 1.11.

Author: christiangnrd

.buildkite/pipeline.yml

@@ -32,10 +32,11 @@ steps:
               - "1.8"
               - "1.9"
               - "1.10"
-              - "nightly"
+              - "1.11"
+              # - "nightly"
           adjustments:
             - with:
-                julia: "nightly"
+                julia: "1.11"
               soft_fail: true
 
   # special tests