Executable.__call__ original semantics were to avoid redirecting process output

to the output stream if return_output was set. This branch undid that and this
commit restores those semantics.

lib/spack/spack/util/executable.py

@@ -100,7 +100,8 @@ def streamify(arg, mode):
             out, err = proc.communicate()
             self.returncode = proc.returncode
 
-            output.write(out)
+            if not return_output:
+                output.write(out)
             error.write(err)
 
             rc = proc.returncode