Move build-mechnical to 12:00 UTC

A single knob controls both FCOS and RHCOS pipelines here. C9S, C10S,
and Rawhide composes seem to run between 04:00 and 05:00 UTC. Meanwhile
RHEL content is generally pushed 09:00 to 16:00 UTC. If there are
changes each build takes approximately 2.5 hours meaning that RHEL
builds would start at 17:00 UTC and should have enough time to complete
by end of day cut-offs, though it'll be tight.

This will likely require additional tweaking in the future when we add
10.2 and 9.8.

Author: sdodson

jobs/build-mechanical.Jenkinsfile

@@ -7,8 +7,8 @@ node {
 
 properties([
     pipelineTriggers([
-        // run every 24h at 10:00 UTC
-        cron("0 10 * * *")
+        // run every 24h at 12:00 UTC
+        cron("0 12 * * *")
     ]),
     buildDiscarder(logRotator(
         numToKeepStr: '100',