build: add workaround for V8 builds

V8's build toolchain is not compatible with Python 3 and the CI job
that tests V8 needs to be run with Python 2. Add a fallback to
`find_executable` from `distutils.spawn` to allow the configure
script to run in the V8 test job.

configure.py

@@ -14,7 +14,12 @@
 import bz2
 import io
 
-from shutil import which
+# Fallback to find_executable from distutils.spawn is a stopgap for
+# supporting V8 builds, which do not yet support Python 3.
+try:
+  from shutil import which
+except ImportError:
+  from distutils.spawn import find_executable as which
 from distutils.version import StrictVersion
 
 # If not run from node/, cd to node/.