Fix issue 5576 (regex matching bug in expr)

Issue 5576 reported a bug in expr, found by the fuzzer. The problem
turns out to be with the regex match operator `:`, which is defined in
POSIX and the GNU manual to match the pattern only when it occurs at
the beginning of the string, i.e., the regex has an implicit `^`
prepended to it. We hadn't been doing that.

tests/by-util/test_expr.rs

@@ -289,6 +289,10 @@ fn test_regex() {
         .args(&["-5", ":", "-\\{0,1\\}[0-9]*$"])
         .succeeds()
         .stdout_only("2\n");
+    new_ucmd!()
+        .args(&["abc", ":", "bc"])
+        .fails()
+        .stdout_only("0\n");
 }
 
 #[test]