SCons: Default num_jobs to max CPUs minus 1 if not specified

This doesn't change the behavior when `--jobs`/`-j` is specified as a
command-line argument or in `SCONSFLAGS`.

The SCons hack used to know if `num_jobs` was set by the user is derived
from the MongoDB setup.

We use `os.cpu_count()` for portability (available since Python 3.4).

With 4 CPUs or less, we use the max. With more than 4 we use max - 1 to
preserve some bandwidth for the user's other programs.

.github/workflows/linux_builds.yml

@@ -212,7 +212,7 @@ jobs:
         if: ${{ matrix.godot-cpp-test }}
         run: |
           cd godot-cpp/test
-          scons target=${{ matrix.target }} -j2
+          scons target=${{ matrix.target }}
           cd ../..
 
       - name: Prepare artifact