expr: Handle '$' at the end of the pattern

Author: frendsick

src/uu/expr/src/syntax_tree.rs

@@ -193,6 +193,8 @@ impl StringOp {
                                     _ => re_string.push_str(r"\$"),
                                 }
                                 re_string.push(backslash);
+                            } else if pattern_chars.peek().is_none() {
+                                re_string.push('$');
                             } else if prev_is_escaped || prev != '\\' {
                                 re_string.push_str(r"\$");
                             } else {