To avoid repeated rebuilds in most cases, and to better enable users to use binaries out of the box, we want to switch to --reuse as the default concretization behavior. This means that when users build, they'll build by default against a) things they already have installed and/or b) binary caches, where possible.

That's a good default CLI behavior, but there are places where we do not want --reuse to be the default -- namely CI. CI should be ensuring that the latest configuration continues to work properly, regardless of any state in the local installation or binary caches. I've changed the default for each of our CI environments here. I am debating whether spack ci should just inherit settings from some common base config scope with "good" settings for CI. That is probably a future PR but it's worth thinking about. All our CI environments currently override a few things -- disabling the view, not doing reuse concretization, etc.

This is draft for now, as we want to fix some outstanding bugs with --reuse first, and also merge #28504 at about the same time. See #24223 for more context.

• change default concretization strategy to reuse
• use reuse: false in all CI environments
• update documentation
• ensure that serious bugs are fixed with reuse before this gets merged