[llvm] [Github][CI] Name Premerge Jobs Experimental (PR #128227)

[Lucile Rose Nihlen via llvm-commits]

================
@@ -25,6 +25,7 @@ concurrency:
 
 jobs:
   premerge-checks-linux:
+    name: Linux Premerge Checks (Experimental)
----------------
lnihlen wrote:

Maybe even more explicit, like "Test Only - Please Ignore Results"?

https://github.com/llvm/llvm-project/pull/128227