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/actions/godot-build/action.yml

@@ -35,5 +35,5 @@ runs:
       run: |
         echo "Building with flags:" ${{ env.SCONSFLAGS }}
         if ! ${{ inputs.tools }}; then rm -rf editor; fi  # Ensure we don't include editor code.
-        scons p=${{ inputs.platform }} target=${{ inputs.target }} tools=${{ inputs.tools }} tests=${{ inputs.tests }} --jobs=2 ${{ env.SCONSFLAGS }}
+        scons p=${{ inputs.platform }} target=${{ inputs.target }} tools=${{ inputs.tools }} tests=${{ inputs.tests }} ${{ env.SCONSFLAGS }}
         ls -l bin/