Make Time.at w/ in: another 1.2x faster

```
$ ips --ruby 'ruby-maybe --yjit' --ruby 'ruby-dev --yjit' -e 'Time.at(1_700_000_000, in: 0)'
ruby 4.1.0dev (2026-07-15T17:09:20Z master 6cd755d) +YJIT +PRISM [arm64-darwin25]
Time.at(1_700_000_000, in: 0):     8.944M i/s (± 0.7%, GC  3.2%)

ruby 4.1.0dev (2026-07-15T17:09:20Z master 6cd755d) +YJIT +PRISM [arm64-darwin25]
Time.at(1_700_000_000, in: 0):    10.925M i/s (± 0.9%, GC  4.0%)

Summary
  ruby ruby-dev --yjit ran
    1.22 ± 0.01 times faster than ruby ruby-maybe --yjit
```

utc_offset_arg checks whether its argument can be to_str'd, which
Integer's (should) not be, so we can fast path it to speed it up

Author: skipkayhil

time.c

@@ -2233,6 +2233,10 @@ invalid_utc_offset(VALUE zone)
 static VALUE
 utc_offset_arg(VALUE arg)
 {
+    if (RB_INTEGER_TYPE_P(arg)) {
+        return arg;
+    }
+
     VALUE tmp;
     if (!NIL_P(tmp = rb_check_string_type(arg))) {
         int n = 0;