### Check existing issues - [x] I've verified that this bug isn't described by any existing issues. ### Bug description With Charmcraft 4.0.1, the [charm-lxd](https://github.com/canonical/charm-lxd) CI was building the primary LXD charm for 24.04 and 22.04 followed by an ancillary charm for 24.04 only one: ``` charmcraft pack -v echo "==> ancillary charm for testing purposes" charmcraft pack -v --project-dir examples/https-client ``` Now with Charmcraft 4.2.0, packing the ancillary charm fails with: ``` Staging charm Priming charm Packing... Packing charm https-client_ubuntu-24.04-amd64.charm Packed https-client_ubuntu-24.04-amd64.charm Launching managed ubuntu 22.04 instance... Creating instance from base instance Starting instance Starting charmcraft, version 4.2.0 Logging execution to '/tmp/charmcraft.log' Platform 'ubuntu-22.04-amd64' not found in the project definition. Detailed information: Valid platforms are: 'ubuntu-24.04-amd64', 'ubuntu-24.04-arm64', 'ubuntu-24.04-armhf', 'ubuntu-24.04-ppc64el', 'ubuntu-24.04-riscv64', 'ubuntu-24.04-s390x'. Failed to run charmcraft in instance ``` The full logs: https://github.com/canonical/charm-lxd/actions/runs/23904129367/job/69708427467 The [main `charmcraft.yaml`](https://github.com/canonical/charm-lxd/blob/b7b57f9660d77c242afc348a26b49b7bed6dcc0e/charmcraft.yaml) and the [anciliary `charmcraft.yaml`](https://github.com/canonical/charm-lxd/blob/b7b57f9660d77c242afc348a26b49b7bed6dcc0e/examples/https-client/charmcraft.yaml) ### Steps to reproduce Build 2 charms consecutively. The first with: ``` type: charm platforms: ubuntu-24.04-amd64: build-on: [ubuntu@24.04:amd64] build-for: [ubuntu@24.04:amd64] ubuntu-22.04-arm64: build-on: [ubuntu@22.04:amd64] build-for: [ubuntu@22.04:amd64] ``` And the second with: ``` type: charm platforms: ubuntu-24.04-amd64: build-on: [ubuntu@24.04:amd64] build-for: [ubuntu@24.04:amd64] ```