Prevent DistutilsOptionError when prefix is indicated in the global e…

…nvironment and --target is used. Fixes pypa#4106.

src/pip/_internal/locations.py

@@ -157,7 +157,7 @@ def distutils_scheme(dist_name, user=False, home=None, root=None,
     # ideally, we'd prefer a scheme class that has no side-effects.
     assert not (user and prefix), "user={} prefix={}".format(user, prefix)
     i.user = user or i.user
-    if user:
+    if user or home:
         i.prefix = ""
     i.prefix = prefix or i.prefix
     i.home = home or i.home