Reduce nightly build cadence (#64557)

Reduce the nightly release workflow from six runs per day to two runs
per day, at 02:00 and 14:00 UTC. Update both the workflow generator and
generated workflow.

Validated by regenerating workflows with `cargo xtask workflows`.

Release Notes:

- N/A

---------

Co-authored-by: Amp <amp@ampcode.com>

.github/workflows/release_nightly.yml

@@ -6,7 +6,7 @@ env:
   RUST_BACKTRACE: '1'
 on:
   schedule:
-  - cron: 0 */4 * * *
+  - cron: 0 2,14 * * *
   workflow_dispatch: {}
 permissions:
   contents: read

tooling/xtask/src/tasks/workflows/release_nightly.rs

@@ -49,8 +49,8 @@ pub fn release_nightly() -> Workflow {
     named::workflow()
         .with_minimal_permissions()
         .on(Event::default()
-            // Fire 6 times a day
-            .schedule([Schedule::new("0 */4 * * *")])
+            // Fire twice a day at 02:00 and 14:00 UTC
+            .schedule([Schedule::new("0 2,14 * * *")])
             .workflow_dispatch(WorkflowDispatch::default()))
         .concurrency(
             Concurrency::default()