Emit custom runs-on label arrays for self-hosted runners (#387)

[avidenic]

Closes #387.

What

Adds a public RunsOnLabels (string[]) to GitHubActionsAttribute so a generated workflow can target a self-hosted runner pool by label array — runs-on: [self-hosted, linux, x64] — instead of only the fixed GitHubActionsImage enum scalar.

[GitHubActions(
    "deploy",
    GitHubActionsImage.UbuntuLatest, // mandatory; ignored for runs-on when RunsOnLabels is set
    On = [GitHubActionsTrigger.WorkflowDispatch],
    InvokedTargets = [nameof(Deploy)],
    RunsOnLabels = ["self-hosted", "linux", "x64"])]
public partial class Build : NukeBuild { /* ... */ }

deploy:
  name: ubuntu-latest
  runs-on: [self-hosted, linux, x64]
  steps:
    - uses: actions/checkout@v6
    # ...

How

1. GitHubActionsAttribute.RunsOnLabels (string[], defaults empty) → carried through GetJobs onto a matching GitHubActionsJob.RunsOnLabels.
2. GitHubActionsJob.Write branches: non-empty → runs-on: [a, b, c] via JoinCommaSpace(); otherwise the existing scalar runs-on: {Image.GetValue()} path is untouched.
3. Validated at configuration-generation time:
   • requires exactly one image — a multi-image matrix can't be disambiguated by a single label set, so it fails the build with a clear message rather than silently emitting N identical-runner jobs. (The constructor mandates an image regardless; when RunsOnLabels is set the image is ignored for runs-on: and only names the job.)
   • rejects null/empty/whitespace entries, which would otherwise emit malformed YAML such as runs-on: [a, ].

Per the issue, the property is named RunsOnLabels (not the fork's earlier RunsOnOptionalInputs). Job- and step-level runs-on and any GitHubActionsImage enum changes are out of scope.

Backwards compatibility

Purely additive. RunsOnLabels defaults to empty, so existing [GitHubActions(...)] usages generate byte-for-byte identical YAML (no runs-on: change). Non-breaking → target/2026, no breaking-change.

Testing

• runs-on-labels Verify snapshot in ConfigurationGenerationTest — locks the runs-on: [self-hosted, linux, x64] array form; the existing scalar snapshots are unchanged, guarding the fallback path.
• GitHubActionsRunsOnLabelsValidationTest: matrix+labels throws; single-image+labels and matrix-without-labels don't; single label is valid; null/empty/whitespace element throws.
• Full solution ./build.sh Test Pack is green locally, including the Nuke.* transition-shim test projects that consume this public API.

[avidenic]

@ChrisonSimtian build is finally green =)

[ChrisonSimtian]

@ChrisonSimtian build is finally green =)

yeah i merged dennis' fix this morning 😬 happy merging

[avidenic]

@ChrisonSimtian build is finally green =)

yeah i merged dennis' fix this morning 😬 happy merging

Tnx! I do not seem to have an option to merge though.

[ChrisonSimtian]

@ChrisonSimtian build is finally green =)

yeah i merged dennis' fix this morning 😬 happy merging

Tnx! I do not seem to have an option to merge though.

did you rebase? Should show you the merge option once you got everything green ticked. If your other PRs are blocked, just ping me and I'll have another look @avidenic