build service for snapstore and launchpad integration is not building for arm64 snap with shorthand architecture definition

Hi Launchpad team,

WE have a snap project [1] that uses core22 base and has architecture requirement defined in the following format:
```
architectures:
  - amd64
  - arm64
```

According to snapcraft's documentation [2], this is equivalent to:
```
architectures:
  - build-on: amd64
    build-for: amd64
  - build-on: arm64
    build-for: arm64
```
which should build on amd64 to produce an amd64 snap and also build on arm64 for an arm64 snap.

However, with the build service for snapstore and github integration [3], we only see amd64 snap being built, not the arm64 one. I was wondering the reason for such a strange behavior, and would appreciate your help on investigating this issue. Thanks!

[1]: https://github.com/canonical/prometheus-juju-exporter
[2]: https://snapcraft.io/docs/architectures
[3]: https://snapcraft.io/build