Use Packse for workspace build constraints test (#20261)
Replace the live `json-merge-patch` sdist in
`sync_workspace_member_build_constraints` with Packse’s generated `a`
sdist. The test still verifies that workspace build constraints apply to
a selected member’s transitive build dependency and remain recorded in
the lockfile.
In [CI](https://github.com/astral-sh/uv/actions/runs/28971212744), JUnit
time fell from 1.986s to 1.118s on Linux (1.8x faster) and from 6.252s
to 3.607s on Windows (1.7x faster).
Co-authored-by: Charlie Marsh <charliemarsh@openai.com>
