[ci.jenkins.io] use Spot instances for agents

[Stéphane MERLE]

Hi dear contributors,

To be able to use our credits longer we defaulted to using AWS Spot instances. These instances provides the same hardware at a cheaper price, but they can be interrupted at any moment if AWS needs capacity for "normal" instances.

Please, be aware that this may impact your build time, even if we chose a machine type with the lowest interruption frequency of interruption (see https://github.com/jenkins-infra/jenkins-infra/pull/3965).

As such, we have set up a retry process that will retry automatically your builds if there is an agent failure. It runs twice on spot instances and falls back to "non Spot" instances the third time, for the plugin builds (see https://github.com/jenkins-infra/helpdesk/issues/4598).

For the infra team.

Stéphane MERLE