Add reusable workflow for build and tests on PR labels

[farshidtz]

Changes between the newly added reusable workflow and workflows used on existing inference snaps:

• init-build-script is deprecated. It is hardcoded to download-models.sh and the caller repo can expect that it will be run if provided. Future version of this reusable workflow will instead run a specific make recipe to download models.
• The build pre-checks applies to both build and test triggers. If a build is detected as present, subsequent requests to build and test get skipped. This change was done primarily to simplify the job conditions. To force a rebuild, one can still delete the workflow log or push an empty commit.
• Support for custom labels has been dropped. Caller must now use trigger-build and trigger-tests labels.

Example caller workflow:

name: PR Label

on:
  pull_request:
    types: [ labeled ]

concurrency:
  group: ${{ github.workflow }}-${{ github.event.pull_request.number }}
  cancel-in-progress: true

permissions:
  contents: read
  pull-requests: write
  actions: read

jobs:

  build-test:
    # The applied label is used to select build/test tasks and as job's display name.
    name: ${{ github.event.label.name }}
    uses: canonical/inference-snaps-dev/.github/workflows/reuse-pr-build-test.yaml@v2
    with:
      trigger-label: ${{ github.event.label.name }}
      pr-number: "${{ github.event.pull_request.number }}"
      build-runners: |
        [
          ["amd64", "resolute", "self-hosted"],
          ["arm64", "resolute", "self-hosted"]
        ]
      test-jobs-matrix: |
        [
          {
            "job-queue": "maas-systemtests-amd64",
            "provision-data": "distro: noble",
            "select-engine": "cpu",
            "test-chat-tps": true
          },
          {
            "job-queue": "anbox-nvidia-amd64",
            "provision-data": "distro: noble",
            "install-nvidia-driver-version": 595,
            "select-engine": "nvidia-gpu",
            "test-chat-tps": true,
            "expected-tps": 9.4
          }
        ]
      snap-name: smollm2
    secrets:
      store-credentials: ${{ secrets.STORE_LOGIN_PR }}
      github-token: ${{ secrets.GITHUB_TOKEN }}

Example runs: