### Bug Description In the last few releases, something seems to have changed in the handling of the architectures definition for `core20` snaps. ### To Reproduce If I create a snap with details such as: ```yaml # ... name: foo version: 0.1 base: core20 architectures: - arm64 - amd64 ``` After `snapcraft pack --use-lxd`, the resulting file is `foo_0.1_multi.snap`, where it was previously `foo_0.1.amd64.snap` as I'd expect. If I expand the definition to: ```yaml # ... name: foo version: 0.1 base: core20 architectures: - build-on: [arm64] run-on: [arm64] - build-on: [amd64] run-on: [amd64] ``` Then I get the expected result. My understanding was that the two forms are supposed to be equivalent, and that the resulting `*_multi.snap` is probably a bug? ### Environment Ubuntu 25.04, Snapcraft 8.12.0, building using LXD ### snapcraft.yaml ```yaml see above ``` ### Relevant log output ```shell see above ``` ### Additional context Discovered while troubleshooting this issue: https://github.com/snapcrafters/sublime-text/issues/86