[Github] Explicitly requesting Ubuntu 22.04 for SPIRV test (llvm#122395)

For the same reason as
[llvm#122221](llvm#122221), this fixes
build failure from missing python3.

Author: adam-yang

.github/workflows/spirv-tests.yml

@@ -26,4 +26,4 @@ jobs:
       build_target: check-llvm-codegen-spirv
       projects:
       extra_cmake_args: '-DLLVM_TARGETS_TO_BUILD="" -DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD="SPIRV" -DLLVM_INCLUDE_SPIRV_TOOLS_TESTS=ON'
-      os_list: '["ubuntu-latest"]'
+      os_list: '["ubuntu-22.04"]'