Repair wp/6.4 branch Actions: Remove the redundant Install NVM step

[desrosj]

Part of #82929.

What?

Removes the Install NVM step from .github/workflows/performance.yml.

Why?

nvm is already installed on GitHub-hosted Ubuntu runners, in $HOME/.nvm. The step's own output says so (=> nvm is already installed in /home/runner/.nvm, trying to update using git), so on every run it only downloads and re-runs the nvm v0.39.1 install script over the existing copy.

Nothing else in the workflow uses the step: the performance CLI loads nvm itself with source $HOME/.nvm/nvm.sh, which the preinstalled copy provides.

Upstream

trunk no longer has this step. It was removed in #82160 ("Performance tests: Build once and shard the suites across parallel CI jobs") as part of a larger rework of the workflow, which isn't ported here.

Use of AI Tools

This PR was created by Claude Code under my supervision. All code should be treated as AI-produced and not yet reviewed by a human until this PR is marked Ready for Review.

🤖 Generated with Claude Code

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[desrosj]

Closing: the performance tests workflow is removed from wp/6.4 in #83667, so this pull request has nothing left to change.