[BUG] salt.pkgrepo not handling arm64

[major0]

SaltStack produces ARM64 builds, but salt-formula hard-codes amd64 into the pkgrepo path.

• https://repo.saltproject.io/py3/ubuntu/20.04/arm64/

Ubuntu:
  pkgrepo: 'deb [signed-by=/usr/share/keyrings/salt-archive-keyring.gpg arch=amd64] {{ salt_repo }}/{{ py_ver_repr or 'apt' }}/{{ os_lower }}/{{ osrelease }}/amd64/{{ salt_release }} {{ oscodename }} main'
  pkgrepo_keyring: '{{ salt_repo }}/{{ py_ver_repr or 'apt' }}/{{ os_lower }}/{{ osrelease }}/amd64/{{ salt_release }}/salt-archive-keyring.gpg'

[fuzzy]

This is still impacting arm64 installs. Confirmed at work and at home.