Use `charmcraft remote-build` for s390x and ppc64el

[sed-i]

Currently, to pack a charm, we take runner name as input,

observability/.github/workflows/_charm-release.yaml

Lines 15 to 21 in c13b2b9

	runners:
	description: |
	Json matrix of the runners to use to build and release the charm.
	Example: ["ubuntu-latest","Ubuntu_ARM64_4C_16G_03"]
	type: string
	required: false
	default: '["ubuntu-latest"]'

and currently the workflow is limited to packing for amd64 and arm64, because those are the runners available from github.

observability/.github/workflows/_charm-release.yaml

Lines 87 to 96 in c13b2b9

	- name: Pack charm(s)
	id: pack
	run: |
	cd "${{ inputs.charm-path }}"
	charmcraft pack -v
	charms="$(basename -a ./*.charm | jq -R -s -c 'split("\n")[:-1]')"
	echo "charms=$charms"
	echo "charms=$charms" >> "$GITHUB_OUTPUT"
	- name: Store charm(s)
	uses: actions/upload-artifact@v4

To pack for s390x and ppc64el, we can use charmcraft remote-build.
Currently, the absence of this blocks:

• Add more platforms grafana-agent-operator#337

In this centralized CI, packing charms for s390x and ppc64el should be done also as part of PR quality checks, but as the last step only after integration tests succeed.

observability/.github/workflows/_charm-quality-checks.yaml

Lines 165 to 166 in c13b2b9

	pack-charm:
	name: Pack the charm

Since charmcraft remote-build does not yet support --build-for, the CI should attempt a remote build only if the .platforms section is not empty after removing arm64 and amd64.

References:

• Do not remote-build amd64, arm64 opentelemetry-collector-snap#50
• Retryable, parallel build grafana-agent-snap#98