chore: use hermetic_llvm for macos execution

Simdutf has this check for enabling atomic operations:

```
  #if __cpp_lib_atomic_ref >= 201806L
    #define SIMDUTF_ATOMIC_REF 1
  #endif // __cpp_lib_atomic_ref
```

Which evidently fails on the runner's macos version, which spits out the following error:

```
external/v8/src/builtins/builtins-typed-array.cc:524:28: error: no member named 'atomic_base64_to_binary_safe' in namespace 'simdutf'; did you mean 'base64_to_binary_safe'?
  524 |     simd_result = simdutf::atomic_base64_to_binary_safe(
      |                   ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                            base64_to_binary_safe
external/simdutf/simdutf.h:4369:1: note: 'base64_to_binary_safe' declared here
 4369 | base64_to_binary_safe(const char *input, size_t length, char *output,
      | ^
external/v8/src/builtins/builtins-typed-array.cc:837:35: error: no member named 'atomic_binary_to_base64' in namespace 'simdutf'; did you mean 'binary_to_base64'?
  837 |       simd_result_size = simdutf::atomic_binary_to_base64(
      |                          ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~
      |                                   binary_to_base64
external/simdutf/simdutf.h:4110:8: note: 'binary_to_base64' declared here
 4110 | size_t binary_to_base64(const char *input, size_t length, char *output,
      |        ^
external/v8/src/builtins/builtins-typed-array.cc:525:9: error: cannot initialize a parameter of type 'const char *' with an rvalue of type 'const char16_t *'

```

Signed-off-by: Matt Leon <[email protected]>

Author: leonm1

.github/workflows/test.yml

@@ -181,6 +181,7 @@ jobs:
           os: macos-15
           arch: x86_64
           action: test
+          flags: --config=hermetic-llvm
           cache: true
         - name: 'WAMR interp on Linux/x86_64'
           engine: 'wamr-interp'