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.

Makefile: Dehardcode -j4, SCons defaults to max - 1

(cherry picked from commits cdcd473371 and
c2b35fb226)

.github/workflows/ci.yml

@@ -79,17 +79,17 @@ jobs:
 
       - name: Build godot-cpp (debug)
         run: |
-          scons platform=${{ matrix.platform }} target=debug generate_bindings=yes ${{ matrix.flags }} -j2
+          scons platform=${{ matrix.platform }} target=debug generate_bindings=yes ${{ matrix.flags }}
 
       - name: Build test without rebuilding godot-cpp (debug)
         run: |
           cd test
-          scons platform=${{ matrix.platform }} target=debug ${{ matrix.flags }} build_library=no -j2
+          scons platform=${{ matrix.platform }} target=debug ${{ matrix.flags }} build_library=no
 
       - name: Build test and godot-cpp (release)
         run: |
           cd test
-          scons platform=${{ matrix.platform }} target=release ${{ matrix.flags }} -j2
+          scons platform=${{ matrix.platform }} target=release ${{ matrix.flags }}
 
       - name: Run test GDNative library
         if: ${{ matrix.platform == 'linux' || matrix.platform == 'osx' }}