#62221 Fix some path handling when installing the local environment from the build directory

[johnbillion]

The npm run env:install command runs the install using the code in src regardless of whether LOCAL_DIR is set to build. 99% of the time this isn't a problem because the two codebases are usually functionally identical, but in the case of the performance tests on GitHub Actions the installation needs to run from the build directory because a different version of WordPress is downloaded into build.

This has always been the case but the effect was not noticed until the performance tests were split up in [59749]. This bug is preventing the performance tests for #7333 from running correctly.

Trac ticket: https://core.trac.wordpress.org/ticket/62221
Trac ticket: https://core.trac.wordpress.org/ticket/21022

[github-actions]

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[github-actions]

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[johnbillion]

Committed in https://core.trac.wordpress.org/changeset/59752