release-24.1: roachtest: --use-spot now properly defaults to "no SpotVMs"

[srosenberg]

Backport 1/1 commits from #124258.

/cc @cockroachdb/release

---

In [1], the default of UseSpotVM was reinterpreted to mean the framework decides whether or not to create a spot VM. This may impede local (performance) testing when a potential preemption is unwelcome; i.e., may result in loss of work.

This change reverts the original default of "no SpotVMs". It now interprets --use-spot auto to mean that the framework decides whether or not to create a spot VM, whereas --use-spot never (default) unconditionally disables the use of spot VMs. Furthermore, to allow unconditional creation of spot VMs, pass --use-spot always. Finally, when running in CI, we pass --use-spot auto, at this time, to allow the framework to conditionally create spot VMs, i.e., based on current heuristics.

[1] https://github.com/cockroachdb/cockroach/pull/116778

Epic: none Fixes: #77376 Release note: None

[blathers-crl[bot]]

Thanks for opening a backport.

Please check the backport criteria before merging:

• [ ] Backports should only be created for serious issues or test-only changes.
• [ ] Backports should not break backwards-compatibility.
• [ ] Backports should change as little code as possible.
• [ ] Backports should not change on-disk formats or node communication protocols.
• [ ] Backports should not add new functionality (except as defined here).
• [ ] Backports must not add, edit, or otherwise modify cluster versions; or add version gates.
• [ ] All backports must be reviewed by the owning areas TL and one additional TL. For more information as to how that review should be conducted, please consult the backport policy.

If your backport adds new functionality, please ensure that the following additional criteria are satisfied:

• [ ] There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• [ ] The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• [ ] New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters. State changes must be further protected such that nodes running old binaries will not be negatively impacted by the new state (with a mixed version test added).
• [ ] The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.
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Also, please add a brief release justification to the body of your PR to justify this backport.

[cockroach-teamcity]

This change is 

[srosenberg]

TFTR!