Configure GN to use python3 to exec_script.

Bug: None
Change-Id: Ifdc79cf363e072ee5eb0a713268fe12851c8a87e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/215229
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33735}

.gn

diff --git a/.gn b/.gn
index a2e2a90..9c43afd 100644
--- a/.gn
+++ b/.gn
@@ -11,6 +11,10 @@
 # The location of the build configuration file.
 buildconfig = "//build/config/BUILDCONFIG.gn"
 
+# The python interpreter to use by default. On Windows, this will look
+# for python3.exe and python3.bat.
+script_executable = "python3"
+
 # The secondary source root is a parallel directory tree where
 # GN build files are placed when they can not be placed directly
 # in the source tree, e.g. for third party source trees.