Fix copilot-setup-steps.yml: move env vars to GITHUB_ENV

[Copilot]

Per GitHub docs, the copilot-setup-steps job only honors steps, permissions, runs-on, services, snapshot, and timeout-minutes. The job-level env: block is silently ignored, so COMPOSABLE_MACHINE_SLOWNESS was never being set in the agent environment — risking performance test failures.

• Moved COMPOSABLE_MACHINE_SLOWNESS, DOTNET_CLI_TELEMETRY_OPTOUT, and DOTNET_NOLOGO from job-level env: into a step writing to $GITHUB_ENV

- name: Set environment variables
  run: |
    echo "COMPOSABLE_MACHINE_SLOWNESS=5.0" >> "$GITHUB_ENV"
    echo "DOTNET_CLI_TELEMETRY_OPTOUT=1" >> "$GITHUB_ENV"
    echo "DOTNET_NOLOGO=true" >> "$GITHUB_ENV"

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✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[Copilot]

Pull request overview

Updates the Copilot coding agent setup workflow so the agent reliably receives required environment variables (notably COMPOSABLE_MACHINE_SLOWNESS) by writing them to $GITHUB_ENV, aligning with GitHub Copilot agent workflow constraints.

Changes:

• Removed the job-level env: block from the copilot-setup-steps job.
• Added a dedicated step that exports COMPOSABLE_MACHINE_SLOWNESS, DOTNET_CLI_TELEMETRY_OPTOUT, and DOTNET_NOLOGO via $GITHUB_ENV.