Correct a variable name

.github/workflows/build.yaml

@@ -219,7 +219,7 @@ jobs:
         # GitHub runners' (homebrew'd) gcc's only support support x86-64 prior to macos-14, and only arm64 after.
         # Note: if we attempt to use multiple archs, gcc will warn, but continue, resulting in later failures.
         elif [[ '${{ matrix.cc }}' == gcc-* ]]; then
-          if [[ '${{ matrix.os }}' == 'macos-13' ]]; then arx='x86_64'; else arch='arm64'; fi
+          if [[ '${{ matrix.os }}' == 'macos-13' ]]; then arch='x86_64'; else arch='arm64'; fi
         # Otherwise, default to universal binaries, where possible.
         else arch='arm64;x86_64'; fi
         echo "CMAKE_OSX_ARCHITECTURES=${arch}" | tee -a "$GITHUB_ENV"