sql: mark timezone(string, string) as VolatilityStable

The `timezone(string, string)` function overload was incorrectly marked
as `VolatilityImmutable`. When the second argument a special string,
like 'now', 'today', 'tomorrow', or 'yesterday', the function is
non-immutable because those strings are cast to timestamps which are
dependent on the current clock time.

This commit correctly marks it as `VolatilityStable`.

Release note: None

pkg/sql/sem/builtins/builtins.go

@@ -2416,7 +2416,7 @@ may increase either contention or retry errors, or both.`,
 				return ts.EvalAtTimeZone(ctx, loc)
 			},
 			Info:       "Convert given time stamp with time zone to the new time zone, with no time zone designation.",
-			Volatility: tree.VolatilityImmutable,
+			Volatility: tree.VolatilityStable,
 		},
 		tree.Overload{
 			Types: tree.ArgTypes{