I have the same issue. The use of Metal for my project had to deal with "compiling" an exr skybox file. The workaround is to generate a the compiled skybox file locally (a realityenv file) and check that into your repo. My workaround has a "Run Script" in Xcode right after the compile source phase to do this step and an Xcode Cloud Environment Variable called XCODE_CLOUD.
if [ "$XCODE_CLOUD" = "true" ]; then
exit 0
fi
$DEVELOPER_BIN_DIR/realitytool compile --platform iphoneos --deployment-target 18.0 -o=$PROJECT_DIR/MyProject $PROJECT_DIR/Source/MySkybox.skybox