ci: RISCOS is a bool

.github/workflows/riscos.yml

@@ -23,7 +23,7 @@ jobs:
       run: |
         cmake -S . -B build -G Ninja \
           -DCMAKE_TOOLCHAIN_FILE=/home/riscos/env/toolchain-riscos.cmake \
-          -DRISCOS=ON \
+          -DRISCOS:BOOL=ON \
           -DSDL_GCC_ATOMICS=OFF \
           -DSDL_TESTS=ON \
           -DSDL_INSTALL_TESTS=ON \