[infra] Force Python 3 to be used in build.

This ports https://crrev.com/c/2515787 for V8.

Bug: v8:9871
Change-Id: I3e62b2960bc622f90d433f9731a287ee14319047
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2886880
Auto-Submit: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74502}

.gn

@@ -7,6 +7,10 @@ import("//build/dotfile_settings.gni")
 # 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"
+
 # These are the targets to check headers for by default. The files in targets
 # matching these patterns (see "gn help label_pattern" for format) will have
 # their includes checked for proper dependencies when you run either