Launchpad no longer builds `core24` snaps for `armhf`. Since `snapcraft
remote-build` submits a single request for all platforms, the `armhf`
rejection fails with `snapcraft internal error: BadRequest()` and takes
the `amd64` and `arm64` builds down with it, so nothing gets published.
Dropping 32-bit ARM from `snap/snapcraft.yaml` and the workflow's
`--build-for` restores nightly snap publishing for the remaining
architectures.