build: use bin override if no python in PATH

On systems with no "python" in the PATH, e.g. FreeBSD, we should always
create a python symlink in get_bin_override().

PR-URL: nodejs/node#16241
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>

configure

@@ -1357,7 +1357,9 @@ def get_bin_override():
   # sys.executable. This directory will be prefixed to the PATH, so that
   # other tools that shell out to `python` will use the appropriate python
 
-  if os.path.realpath(which('python')) == os.path.realpath(sys.executable):
+  which_python = which('python')
+  if (which_python and
+      os.path.realpath(which_python) == os.path.realpath(sys.executable)):
     return
 
   bin_override = os.path.abspath('out/tools/bin')