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/exec.gni b/pw_build/exec.gni
index 787d69c..d4cec4f 100644
--- a/pw_build/exec.gni
+++ b/pw_build/exec.gni
@@ -12,7 +12,7 @@
 # License for the specific language governing permissions and limitations under
 # the License.
 
-import("python_script.gni")
+import("python_action.gni")
 
 # Runs a program which isn't in Python.
 #