Pilot runners failover

What does this MR do?

Introduces a single USE_PILOT_RUNNERS toggle (default: true) that makes all runner tags variable-driven. When disabled, jobs fall back to shared/instance runner tags, restoring GitLab Duo.

Files Changed

1. .gitlab/ci/_project_canonical.gitlab-ci.yml — Tag anchors use CI variable references:

.instance-default: $RUNNER_TAG_DEFAULT
.instance-default-docker: $RUNNER_TAG_DOCKER
.instance-2xlarge: $RUNNER_TAG_2XLARGE
.instance-medium: $RUNNER_TAG_MEDIUM
.instance-windows-2019: $RUNNER_TAG_WINDOWS_2019
.instance-windows-2022: $RUNNER_TAG_WINDOWS_2022

2. .gitlab/ci/_common.gitlab-ci.yml — Added toggle and pilot tag defaults:

USE_PILOT_RUNNERS: "true"
RUNNER_TAG_DEFAULT: "functions-pilot-linux-amd64"
RUNNER_TAG_DOCKER: "functions-pilot-linux-amd64"
RUNNER_TAG_2XLARGE: "functions-pilot-linux-amd64"
RUNNER_TAG_MEDIUM: "functions-pilot-linux-amd64"
RUNNER_TAG_WINDOWS_2019: "functions-pilot-windows-amd64"
RUNNER_TAG_WINDOWS_2022: "functions-pilot-windows-amd64"

3. .gitlab/ci/_rules.gitlab-ci.yml — Workflow rule overrides tags when toggle is off:

- if: '$USE_PILOT_RUNNERS == "false"'
  variables:
    RUNNER_TAG_DEFAULT: "gitlab-org"
    RUNNER_TAG_DOCKER: "gitlab-org-docker"
    RUNNER_TAG_2XLARGE: "saas-linux-2xlarge-amd64"
    RUNNER_TAG_MEDIUM: "saas-linux-medium-amd64"
    RUNNER_TAG_WINDOWS_2019: "windows-1809"
    RUNNER_TAG_WINDOWS_2022: "windows-21h1"

4. .gitlab/ci/build.gitlab-ci.yml — Fixed hardcoded tag:

# before
helper images:
  tags:
    - saas-linux-2xlarge-amd64

# after
helper images:
  tags:
    - !reference [.instance-2xlarge]

Tag Mapping

Variable	Pilot (default)	Fallback (USE_PILOT_RUNNERS=false)
RUNNER_TAG_DEFAULT	functions-pilot-linux-amd64	gitlab-org
RUNNER_TAG_DOCKER	functions-pilot-linux-amd64	gitlab-org-docker
RUNNER_TAG_2XLARGE	functions-pilot-linux-amd64	saas-linux-2xlarge-amd64
RUNNER_TAG_MEDIUM	functions-pilot-linux-amd64	saas-linux-medium-amd64
RUNNER_TAG_WINDOWS_2019	functions-pilot-windows-amd64	windows-1809
RUNNER_TAG_WINDOWS_2022	functions-pilot-windows-amd64	windows-21h1

Why was this MR needed?

Pilot runners were tagged with gitlab-org to match existing jobs. Shared/instance runners were disabled to ensure deterministic routing, which broke GitLab Duo (needs instance runners with gitlab--duo tag). This MR allows re-enabling shared/instance runners since pilot runners now have unique functions-pilot-* tags.

What's the best way to test this MR?

1. Default behavior: verify pipeline jobs are picked up by functions-pilot-* runners
2. Set USE_PILOT_RUNNERS=false as a CI/CD variable and verify jobs fall back to shared/instance runners

What are the relevant issue numbers?

step-runner#425 (closed)