Add support for arm64 builds for charm-local-users

[samuelallan72]

This ports the release.yaml changes from charm-local-users to here. :)

[samuelallan72]

From my refactor #38 :

type any will be required if we want template variables of other types - for example a list of architectures to run on (useful for multiarch releases coming soon)

Unfortunately it turned out this didn't actually work:

The given value is not suitable for var.templates declared at
variables.tf:22,1-21: cannot find a common base type for all elements.

So we need to use string values for everything with the current implementation anyway.

[chanchiwai-ray]

A question, otherwise LGTM