Update clang version in pull request workflow (#226)

Author: jwaldrop107

.github/workflows/pull_request.yaml

@@ -32,10 +32,10 @@ jobs:
   test_library:
     uses: NWChemEx/.github/.github/workflows/test_nwx_library.yaml@master
     with:
-      compilers: '["gcc-14", "clang-19"]'
+      compilers: '["gcc-14", "clang-18"]'
 
   test_library_with_sigma:
     uses: NWChemEx/.github/.github/workflows/test_nwx_library.yaml@master
     with:
-      compilers: '["gcc-14", "clang-19"]'
+      compilers: '["gcc-14", "clang-18"]'
       repo_toolchain: ".github/enable_sigma.cmake"