Use bhttp from latest/edge

[freyes]

the bhttp snap is available in latest/edge for the architectures we care
about. This enables the building of charm-tools as a snap in non-amd64
architectures.

Closes #666

Checklist

• Are all your commits [logically] grouped and squashed appropriately?
• Does this patch have code coverage?
• Does your code pass make test?

[freyes]

this is 'snap info' in a aarch64 machine

$ uname -a
Linux rpi4 6.5.0-1005-raspi #7-Ubuntu SMP PREEMPT_DYNAMIC Sun Oct  8 08:06:18 UTC 2023 aarch64 aarch64 aarch64 GNU/Linux
$ snap info bhttp
name:      bhttp
summary:   Macaroon-aware HTTP command line client
publisher: rog
store-url: https://snapcraft.io/bhttp
license:   unset
description: |
  The bhttp snap provides the bhttp command line tool which
  replicates the command-line interface of httpie but will also
  automatically obtain macaroon-based authentication when
  required by the server.
commands:
  - bhttp
snap-id:      Lyh4DOELsVckHXsTHgNFkLNGoQjT3Um5
tracking:     latest/edge
refresh-date: 6 days ago, at 17:04 -03
channels:
  latest/stable:    –                                 
  latest/candidate: –                                 
  latest/beta:      –                                 
  latest/edge:      0+git.8623f22 2019-06-13 (32) 3MB -
installed:          0+git.8623f22            (32) 3MB -