After #41373, where we stopped considering the source directory to be the stage for develop builds,
we resumed *deleting* the stage even after a successful build.
We don't want this for develop builds because developers need to iterate; we should keep the artifacts
unless they explicitly run `spack clean`.
Now:
- [x] Build artifacts for develop packages are not removed after a successful install
- [x] They are also not removed before an install starts, i.e. develop packages always
reuse prior artifacts, if available.
- [x] They can be deleted in any other context, e.g. by running `spack clean --stage`
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