ci:linux: flang 20

flang 21 could not be detected by CMake due to
/usr/bin/ld: cannot find -lflang_rt.runtime: No such file or directory
On Github Actions Ubuntu 24.04 runner

Author: scivision

.github/workflows/ci.yml

@@ -92,7 +92,7 @@ jobs:
 
     strategy:
       matrix:
-        llvm-version: [21]
+        llvm-version: [20]
 
     env:
       CC: clang-${{ matrix.llvm-version }}