Simplify NanoGUI workaround in GitHub CI (#2332)

This changelist simplifies our workaround for NanoGUI CMake settings in our GitHub CI, defining CMAKE_POLICY_VERSION_MINIMUM at CMake generation time.

Author: jstone-lucasfilm

.github/workflows/main.yml

@@ -172,17 +172,12 @@ jobs:
       if: matrix.python != 'None'
       run: pip install setuptools
 
-    - name: Install CMake
-      uses: lukka/get-cmake@28983e0d3955dba2bb0a6810caae0c6cf268ec0c # v4.0.0
-      with:
-        cmakeVersion: 3.26
-
     - name: Run Clang Format
       if: matrix.clang_format == 'ON'
       run: find source \( -name *.h -o -name *.cpp -o -name *.mm -o -name *.inl \) ! -path "*/External/*" ! -path "*/NanoGUI/*" | xargs clang-format -i --verbose
 
     - name: CMake Generate
-      run: cmake -S . -B build -DMATERIALX_BUILD_PYTHON=ON -DMATERIALX_BUILD_VIEWER=ON -DMATERIALX_BUILD_GRAPH_EDITOR=ON -DMATERIALX_BUILD_TESTS=ON -DMATERIALX_TEST_RENDER=OFF -DMATERIALX_WARNINGS_AS_ERRORS=ON ${{matrix.cmake_config}}
+      run: cmake -S . -B build -DMATERIALX_BUILD_PYTHON=ON -DMATERIALX_BUILD_VIEWER=ON -DMATERIALX_BUILD_GRAPH_EDITOR=ON -DMATERIALX_BUILD_TESTS=ON -DMATERIALX_TEST_RENDER=OFF -DMATERIALX_WARNINGS_AS_ERRORS=ON "-DCMAKE_POLICY_VERSION_MINIMUM=3.5" ${{matrix.cmake_config}}
 
     - name: CMake Build
       run: cmake --build build --target install --config Release --parallel 2