CI: test O= builds too

O= builds do not work at the moment, but we will fix them soon.

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

Author: ojeda

.github/workflows/ci.yaml

@@ -9,6 +9,7 @@ jobs:
       matrix:
         mode: [debug, release]
         module: [builtin, loadable]
+        outputdir: [srcdir, builddir]
 
     steps:
       # Setup
@@ -22,7 +23,10 @@ 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 == 'srcdir'
+        run: make         CC=clang-10 LLVM_CONFIG_PATH=llvm-config-10 -j3
+      - if: matrix.outputdir == 'builddir'
+        run: make O=build CC=clang-10 LLVM_CONFIG_PATH=llvm-config-10 -j3
 
       # Run
       - if: matrix.module == 'builtin'