ci: pin cmake to 3.31.6

[Saadnajmi]

Summary:

Currently all Hermes slices in PR are failing to build (but build locally).

2025-09-17T00:55:31.8097360Z CMake Error at CMakeLists.txt:42 (cmake_policy):
2025-09-17T00:55:31.8097720Z   Policy CMP0026 may not be set to OLD behavior because this version of CMake
2025-09-17T00:55:31.8098290Z   no longer supports it.  The policy was introduced in CMake version 3.0.0,
2025-09-17T00:55:31.8098600Z   and use of NEW behavior is now required.
2025-09-17T00:55:31.8098750Z 
2025-09-17T00:55:31.8098940Z   Please either update your CMakeLists.txt files to conform to the new
2025-09-17T00:55:31.8099330Z   behavior or use an older version of CMake that still supports the old
2025-09-17T00:55:31.8099700Z   behavior.  Run cmake --help-policy CMP0026 for more information.
2025-09-17T00:55:31.8099900Z 
2025-09-17T00:55:31.8099900Z 
2025-09-17T00:55:31.8101310Z CMake Error: CMake was unable to find a build program corresponding to "Unix Makefiles".  CMAKE_MAKE_PROGRAM is not set.  You probably need to select a different build tool.
2025-09-17T00:55:31.8101980Z CMake Error: CMAKE_C_COMPILER not set, after EnableLanguage
2025-09-17T00:55:31.8102350Z CMake Error: CMAKE_CXX_COMPILER not set, after EnableLanguage

This is because GitHub Actions recently updated the macOS images to cmake 4 (actions/runner-images#12934). Per their comment, use their example script to pin to a lower version

Test Plan:

CI should pass