Merge pull request opencv#19208 from alalek:js_build_reuse_python

Author: alalek

platforms/js/build_js.py

@@ -77,7 +77,9 @@ def clean_build_dir(self):
             rm_one(d)
 
     def get_cmake_cmd(self):
-        cmd = ["cmake",
+        cmd = [
+            "cmake",
+            "-DPYTHON_DEFAULT_EXECUTABLE=%s" % sys.executable,
                "-DENABLE_PIC=FALSE", # To workaround emscripten upstream backend issue https://github.com/emscripten-core/emscripten/issues/8761
                "-DCMAKE_BUILD_TYPE=Release",
                "-DCMAKE_TOOLCHAIN_FILE='%s'" % self.get_toolchain_file(),