Use vpython3 as the default interpreter for gn.

Change-Id: If775d156a3c069f29635a14334becee7b1f010ac
Bug: None
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/346460
Commit-Queue: Jeremy Leconte <jleconte@google.com>
Reviewed-by: Emil Lundmark <lndmrk@webrtc.org>
Reviewed-by: Jeremy Leconte <jleconte@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#42025}

.gn

diff --git a/.gn b/.gn
index b886ea8..5de9689 100644
--- a/.gn
+++ b/.gn
@@ -12,8 +12,8 @@
 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"
+# for vpython3.exe and vpython3.bat.
+script_executable = "vpython3"
 
 # The secondary source root is a parallel directory tree where
 # GN build files are placed when they can not be placed directly