Bug 1476147 - Use path.isfile() instead of patch.exists() in which(). r=glandium

This excludes directories, and returns true only if it's an executable file.

Differential Revision: https://phabricator.services.mozilla.com/D3366

--HG--
extra : moz-landing-system : lando

python/mozboot/mozboot/base.py

@@ -305,7 +305,7 @@ class BaseBootstrapper(object):
         """
         for path in os.environ['PATH'].split(os.pathsep):
             test = os.path.join(path, name)
-            if os.path.exists(test) and os.access(test, os.X_OK):
+            if os.path.isfile(test) and os.access(test, os.X_OK):
                 return test
 
         return None