Fixes bug closing Executable file io-streams (#1890)

Input/output/error streams not directed to None or 'str' were not being closed
because `close()` method was being called on the argument value (a string type)
instead of the open file descriptor object.

lib/spack/spack/util/executable.py

@@ -185,11 +185,11 @@ def streamify(arg, mode):
 
         finally:
             if close_ostream:
-                output.close()
+                ostream.close()
             if close_estream:
-                error.close()
+                estream.close()
             if close_istream:
-                input.close()
+                istream.close()
 
     def __eq__(self, other):
         return self.exe == other.exe