Fix non x86 builds of charm-tools itself

[fnordahl]

Please review commit by commit.

• setup: Update charm-tools pin set

  • Update the dependency pin set so that dependencies can be consumed from versions from the Ubuntu distribution wherever possible.

• snap: Ensure build succeeds on non-x86 architectures

  • Update snap recipe so that building charm-tools itself succeeds on non-x86 architectures.

  • With reproducibility and maintainability of the snap in mind, make use of Python dependencies from Ubuntu distribution packages wherever possible.

  • For any dependencies not available in the Ubuntu distribution, we request a build from source as this will ensure that build-package requirements are uncovered at a early stage in the development process, preferably already in the gate check.

  • Drop the PYTHON related build-environment variables as these would mandate installing all build dependencies in the snap, which is not what we want. Instead reference these variables when needed in the build recipe.

[fnordahl]

Binary artifacts available for review in the latest/edge/fix-non-x86-builds snap branch.

Build logs per architecture: charm_s390x.txt charm_ppc64el.txt charm_arm64.txt charm_amd64.txt