Fix install.py on windows (#24605)

This should unblock the emscripten-releases roller.

Author: sbc100

test/test_other.py

@@ -16289,3 +16289,12 @@ def test_parsetools_make_removed_fs_assert(self):
 
     self.emcc(test_file('hello_world.c'), ['-lidbfs.js'], output_filename='hello_world.js')
     self.assertNotContained(removed_fs_assert_content, read_file('hello_world.js'))
+
+  @is_slow_test
+  @crossplatform
+  def test_install(self):
+    self.run_process([PYTHON, path_from_root('tools/install.py'), 'newdir'])
+    self.assertExists('newdir/emcc')
+    # Some files, such as as maintenance tools should not be part of the
+    # install.
+    self.assertNotExists('newdir/tools/maint/')

tools/install.py

@@ -85,7 +85,7 @@ def copy_emscripten(target):
 
 
 def npm_install(target):
-  subprocess.check_call(['npm', 'ci', '--omit=dev'], cwd=target)
+  subprocess.check_call([shutil.which('npm'), 'ci', '--omit=dev'], cwd=target)
 
 
 def main():