Fixed index_url logic in develop compatibility shim.

Closes #4966

Author: jaraco

newsfragments/4966.bugfix.rst

@@ -0,0 +1 @@
+Fixed index_url logic in develop compatibility shim.

setuptools/command/develop.py

@@ -34,7 +34,7 @@ def run(self):
             + ['--no-deps'] * self.no_deps
             + ['--user'] * self.user
             + ['--prefix', self.prefix] * bool(self.prefix)
-            + ['--index-url', self.index_url] * bool(self.prefix)
+            + ['--index-url', self.index_url] * bool(self.self.index_url)
         )
         subprocess.check_call(cmd)