Ubuntu 20.04 Does not support i386, so attempting to build the Snap for
i386 will either fail or be non-runnable on an i386 Linux distro.
* Disable i386 build in `snapcraftbuild.yml` GitHub Action workflow
* Disable i386 build when testing PRs for consistency
Signed-off-by: Daniel Llewellyn <daniel@snapcraft.ninja>