build: skip emulator tests for forks

[hallvictoria]

Description

Skip emulator test jobs for pull requests originating from forks.

Emulator tests depend on secret variables that Azure Pipelines intentionally withholds from fork builds. Without those values, unresolved variable references reach the emulator SDKs and cause misleading credential and connection-string failures.

This change conditionally skips the shared emulator test job when System.PullRequest.IsFork is True, while preserving the full emulator test matrix for trusted PRs, CI builds, and scheduled runs.

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Pull Request Checklist

Host-Worker Contract

• Does this PR impact the host-worker contract (e.g., gRPC messages, shared interfaces)?
  • If yes, have the changes been applied to:
    • azure_functions_worker (Python <= 3.12)
    • proxy_worker (Python >= 3.13)
  • If no, please explain why:

Worker Execution Logic

• Does this PR affect worker execution logic (e.g., function invocation, bindings, lifecycle)?
  If yes, please answer the following:

Python Version Coverage

• Does this change apply to both Python <=3.12 and 3.13+?
• If yes, have the changes been made to:
  • azure_functions_worker (Python <= 3.12)
  • runtimes/v1 / runtimes/v2 (Python >= 3.13)
• If no, please explain why:

Programming Model Compatibility (for Python 3.13+)

• Does this change apply to both:
  • V1 programming model (runtimes/v1)?
  • V2 programming model (runtimes/v2)?
• Explanation (if limited to one model):