[GN] Explicitly set script_executable to python3

When we don't have python3 available as an executable i.e. on
windows, ensure that our python executable is python 3.

b/62677273

Change-Id: I871d2c879761381a8c0234d449ab5841df4f4b46

.gn

@@ -14,3 +14,8 @@
 
 # The location of the build configuration file.
 buildconfig = "//starboard/build/config/BUILDCONFIG.gn"
+
+# We have `python3` if we're in a container or on buildbot, so use that.
+if (getenv("IS_DOCKER") == "1" || getenv("BUILDBOT_PROJECT") != "") {
+  script_executable = "python3"
+}

starboard/build/config/BUILDCONFIG.gn

@@ -12,6 +12,9 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+_is_python2 = exec_script("//build/util/is_python2.py", [], "json")
+assert(!_is_python2, "`python` must resolve to Python 3 when building with GN.")
+
 declare_args() {
   is_clang = true