rbenv-prefix: do not silence rbenv-which for system version

This suppressed any output when using RBENV_DEBUG=1 and does not really
hurt to have in the unlikely case that it should fail; you would get
two error messages now:

rbenv: ruby: command not found
rbenv: system version not found in PATH

test/prefix.bats

@@ -38,5 +38,8 @@ OUT
 
 @test "prefix for invalid system" {
   PATH="$(path_without ruby)" run rbenv-prefix system
-  assert_failure "rbenv: system version not found in PATH"
+  assert_failure <<EOF
+rbenv: ruby: command not found
+rbenv: system version not found in PATH"
+EOF
 }