upip: expandhome(): Call uos.getenv() only if there's something to expand.

To work on baremetal targets without getenv().

upip/upip.py

@@ -89,6 +89,7 @@ def install_tar(f, prefix):
     return meta
 
 def expandhome(s):
+    if "~/" in s:
         h = os.getenv("HOME")
         s = s.replace("~/", h + "/")
     return s