remote-build doesn't support shorthand architecture format

[Guillaume Beuzeboc]

In the architecture documentation: https://forum.snapcraft.io/t/architectures/4972

It's mentioned:

> architectures:
> - build-on: [amd64]
>
> The shorthand format will also produce the same result:
>
> architectures:
> - amd64

When using remote build and using the semi-complete architecture with "build-on" remote-build is considering it. On the contrary, when using the shorthand architecture format, remote-build is ignoring it.
If both syntax are producing the same result, it should be the same for remote-build.

Using the shorthand architecture format allows us to specify the --build-on flag on top of the architecture definition with remote-build while the help specifically mention:

>If not specified in the snapcraft.yaml file, the list of
    architectures to build can be set using the --build-on option.
    If both are specified, an error will occur.

snapcraft 7.5.3

[Callahan Kovacs (mr-cal)]

Thanks for the report @gbeuzeboc. I can confirm this behavior.

This bug is from my mistake. When I implemented architecture support for core22 snaps in snapcraft 7, I added the shorthand format but we hadn't agreed with the Launchpad team to support the shorthand format.

I think the path forward is to continue to support the shorthand format but remove it from the documentation to discourage its usage.

[Callahan Kovacs (mr-cal)]

https://github.com/snapcore/snapcraft/pull/4436