Remote-build on core20 projects do not understand `--build-for`

[bepri]

Bug Description

When building a core20 project not using the architecture shorthand, --build-for produces the following error:

Error: Cannot use --build-on, architecture list is already set in snapcraft.yaml.

To Reproduce

Run snapcraft remote-build --build-for amd64 with the attached project.

Environment

snapcraft 8.12.0, ubuntu 25.04

snapcraft.yaml

name: testing # you probably want to 'snapcraft register <name>'
base: core20 # the base snap is the execution environment for this snap
version: "0.1" # just for humans, typically '1.2+git' or '1.3.2'
summary: Single-line elevator pitch for your amazing snap # 79 char long summary
description: |
  This is my-snap's description. You have a paragraph or two to tell the
  most important story about your snap. Keep it under 100 words though,
  we live in tweetspace and your description wants to look good in the snap
  store.

grade: devel # must be 'stable' to release into candidate/stable channels
confinement: devmode # use 'strict' once you have the right plugs and slots

architectures:
  - build-on: [arm64]
    run-on: [arm64]
  - build-on: [amd64]
    run-on: [amd64]

parts:
  my-part:
    # See 'snapcraft plugins'
    plugin: nil

Relevant log output

n/a

Additional context

No response