Keep all packed charms, if there are more than one

[sed-i]

When charmcraft.yaml has multiple bases, then multiple *.charm files are created. Before this PR, build_charm kept only one charm file:

find . -type f -name "*.charm"
./.tox/integration/tmp/pytest/test-juju-info-l4uc0/charms/grafana-agent_ubuntu-20.04-amd64.charm

With this PR we'd keep all *.charm files.

find . -type f -name "*.charm" 
./.tox/integration/tmp/pytest/test-juju-info-9fz20/charms/grafana-agent_ubuntu-20.04-amd64.charm
./.tox/integration/tmp/pytest/test-juju-info-9fz20/charms/grafana-agent_ubuntu-22.04-amd64.charm

Usage example:

• https://github.com/canonical/grafana-agent-operator/pull/9

[sed-i]

Could you please take a look @addyess @neoaggelos @carlcsaposs-canonical ?

[addyess]

@sed-i seems reasonable. If you can clean up the line issues i'd be okay to merge + release a new version.

[sed-i]

@addyess anything immediately obvious why itests fail?