CI: test O= builds too

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

.github/workflows/ci.yaml

@@ -12,6 +12,11 @@ jobs:
       matrix:
         mode: [debug, release]
         module: [builtin, loadable]
+        outputdir: [src, build]
+
+    env:
+      BUILD_DIR: build
+      MAKE_FLAGS: CC=clang-10 LLVM_CONFIG_PATH=llvm-config-10 -j3
 
     steps:
       # Setup
@@ -26,7 +31,13 @@ jobs:
       - run: cp .github/workflows/kernel-${{ matrix.mode }}.config .config
       - if: matrix.module == 'loadable'
         run: sed -i -E 's/^(CONFIG_RUST_EXAMPLE=)(y)$/\1m/g' .config
-      - run: make CC=clang-10 LLVM_CONFIG_PATH=llvm-config-10 -j3
+      - if: matrix.outputdir == 'src'
+        run: make ${MAKE_FLAGS}
+      - if: matrix.outputdir == 'build'
+        run: |
+          mkdir ${BUILD_DIR}
+          mv .config ${BUILD_DIR}/.config
+          make ${MAKE_FLAGS} O=${BUILD_DIR}
 
       # Run
       - if: matrix.module == 'builtin'