feat: add chisel as a build snap when stage-packages contain slices

[linostar]

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This PR aims to stop chisel from being unnecessarily installed in the Rockcraft snap.

It does that by having chisel as a build snap only when the following conditions are met:

• there is at least one part that has slices in its stage-packages
• there is no part that has chisel in its build-snaps

[linostar]

@canonical/starcraft-reviewers This is ready for review.