pw_build: Rename python_script.gni to python_action.gni

Rename the python_script.gni file to python_action.gni to prepare to
rename the pw_python_script template.

An alias .gni file is created in separate change so that Git tracks this
as a file move rather than an update.

Change-Id: I812cf3bd73909593c5369c7437da16529e1a375a
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/22000
Reviewed-by: Alexei Frolov <frolv@google.com>
diff --git a/pw_build/docs.rst b/pw_build/docs.rst
index 9882983..dc101d4 100644
--- a/pw_build/docs.rst
+++ b/pw_build/docs.rst
@@ -197,7 +197,7 @@
 
 .. code::
 
-  import("$dir_pw_build/python_script.gni")
+  import("$dir_pw_build/python_action.gni")
 
   pw_python_script("postprocess_main_image") {
     script = "py/postprocess_binary.py"