uv build: add --keep-on-error

[da-x]

While debugging build failures, it is sometimes useful to look at the build directory for logs. Let's allow not deleting it.

[zanieb]

Should this just be the default behavior?

[konstin]

This is a neat idea! Assuming that the operating system reliably cleans up those directories on reboot, I'd actually prefer preserving temporary directories on build failure unconditionally.

[da-x]

I don't think we should change the default, as this applies to the ~/.cache/uv/build-v0 directory which the OS does not clean up.

[konstin]

Makes sense!

[zanieb]

We could clean it with uv cache clean or uv cache prune though and we could overwrite it on a subsequent build so there's only a single build directory for a given project version. I'm sort of partial to retaining information on error unless there are major concerns about disk use?