CI: macOS Python Executable

Ensure we always use the same `python3` executable, as specified
by the `PATH` priority.

.github/workflows/macos.yml

@@ -63,6 +63,9 @@ jobs:
           ccache-macos-appleclang-
     - name: build WarpX
       run: |
+        python3 -m pip install --upgrade pip
+        python3 -m pip install --upgrade pip setuptools wheel
+
         cmake -S . -B build_dp         \
           -DCMAKE_VERBOSE_MAKEFILE=ON  \
           -DWarpX_EB=ON                \
@@ -72,14 +75,13 @@ jobs:
 
         cmake -S . -B build_sp         \
           -DCMAKE_VERBOSE_MAKEFILE=ON  \
+          -DPython_EXECUTABLE=$(which python3) \
           -DWarpX_EB=ON                \
           -DWarpX_PYTHON=ON            \
           -DWarpX_OPENPMD=ON           \
           -DWarpX_openpmd_internal=OFF \
           -DWarpX_PRECISION=SINGLE
         cmake --build build_sp -j 3
-
-        python3 -m pip install --upgrade pip setuptools wheel
         cmake --build build_sp --target pip_install
 
     - name: run pywarpx