Use system GCC instead of conda Clang

The conda-forge Clang is built with newer CPU instructions that
cause illegal instruction errors on older CPUs. Override to use
system GCC and remove mold linker flags.

.gitea/workflows/build.yml

@@ -57,7 +57,14 @@ jobs:
           pixi run ccache -p
 
       - name: Configure (CMake)
-        run: pixi run configure-release
+        shell: bash
+        run: |
+          # Use system GCC instead of conda Clang to avoid illegal instruction errors
+          pixi run cmake --preset conda-linux-release \
+            -DCMAKE_C_COMPILER=gcc \
+            -DCMAKE_CXX_COMPILER=g++ \
+            -DCMAKE_EXE_LINKER_FLAGS="" \
+            -DCMAKE_SHARED_LINKER_FLAGS=""
 
       - name: Build
         run: pixi run cmake --build build/release -j16