Fix UV benchmark pipeline: Linux PATH, venv dot-source params, and install pattern

[Copilot]

Three bugs in the UV-based benchmark pipeline prevented it from running successfully on Linux CI agents.

Bugs fixed

• install-uv.yml — Linux PATH never updated: $HOME/.local/bin prepend step was conditioned on Darwin only; the CI pool is Linux, so uv was installed but not on PATH. Condition expanded to or(Linux, Darwin).

• Invoke-CopilotBenchmarks.ps1 — broken venv dot-source: Script dot-sourced Create-Venv.ps1 / Activate-Venv.ps1 passing -RepoRoot, which neither script's param() block declares → immediate runtime error. Removed the dead dot-source calls and unused $repoRoot variable.

• Invoke-CopilotBenchmarks.ps1 — wrong UV install/run pattern: uv pip install requires an active venv or --system; uv run 'msbench-cli' doesn't invoke an installed CLI tool. Replaced with the correct UV-native pattern:

# Before (broken)
uv pip install msbench-cli
uv run 'msbench-cli' version
uv run 'msbench-cli' @runArgs

# After
uv tool install msbench-cli   # isolated env, binary landed in ~/.local/bin (now on PATH)
msbench-cli version
msbench-cli @runArgs

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