Provide `armhf` `lxd-agent` in `arm64` snap to enable `lxc exec` into 32 bit guests from 64 bit host

[simondeziel]

The ability to launch armhf guests on arm64 host landed in #14077. It would be nice to also have the ability to lxc exec into those 32 bit guests.

[tomponline]

https://github.com/lxc/incus/pull/427 might help with this