mktemp: error on path separator in template prefix

Correct the error that arises from a path separator in the prefix
portion of a template argument provided to `mktemp`. Before this
commit, the error message was incorrect:

    $ mktemp -t a/bXXX
    mktemp: failed to create file via template 'a/bXXX': No such file or directory (os error 2) at path "/tmp/a/bege"

After this commit, the error message is correct:

    $ mktemp -t a/bXXX
    mktemp: invalid template, 'a/bXXX', contains directory separator

The code was failing to check for a path separator in the prefix
portion of the template.

tests/by-util/test_mktemp.rs

@@ -2,6 +2,8 @@
 
 use crate::common::util::*;
 
+use uucore::display::Quotable;
+
 use std::path::PathBuf;
 use tempfile::tempdir;
 
@@ -482,3 +484,15 @@ fn test_respect_template_directory() {
     assert_matches_template!(template, filename);
     assert!(at.file_exists(filename));
 }
+
+/// Test that a template with a path separator is invalid.
+#[test]
+fn test_template_path_separator() {
+    new_ucmd!()
+        .args(&["-t", "a/bXXX"])
+        .fails()
+        .stderr_only(format!(
+            "mktemp: invalid template, {}, contains directory separator\n",
+            "a/bXXX".quote()
+        ));
+}