### Bug Description When mis-inputting the `platforms.<platform>.build-on` rockcraft.yaml field as a single string instead of a list of strings, the resulting error message makes it sound like the `<platform>:` dict should be the one which is a list. ### To Reproduce Please `rockcraft build` the below-listed `rockcraft.yml`. ### Environment * Ubuntu 22.04 * x86_64 VM on VMware vSphere FWIW * rockcraft 1.5.2 from snap (`snap install rockcraft --classic`) ### rockcraft.yaml ```shell # Basically the example from the below tutorial with a modified `platforms` section: # https://documentation.ubuntu.com/rockcraft/en/latest/tutorials/chisel/#project-setup platforms: arm64: # NOTE: this should be a list of strings: build-on: amd64 ### Unmodified rest of spec: name: chiselled-hello summary: Hello world from Chisel slices description: A "bare" rock containing the "hello" package binaries from Chisel slices. license: Apache-2.0 version: "latest" base: bare build_base: "ubuntu@22.04" parts: hello: plugin: nil stage-packages: - hello_bins ``` ### Relevant log output ```shell $ rockcraft build --debug Bad rockcraft.yaml content: - error for platform entry 'arm64': value is not a valid list (in field 'platforms') For more information, check out: https://documentation.ubuntu.com/rockcraft/en/stable/reference/rockcraft.yaml ```