linting: Do pnpm install in Jetpack E2E dir before eslint

[anomiex]

Changes proposed in this Pull Request:

We don't include that as part of the workspace so normal development
doesn't need to install all of playwright and so on. But that also means
the root-level pnpm install doesn't process it, which can make eslint
be unable to resolve imports.

Jetpack product discussion

Came up in code review on #20326

Does this pull request change what data or activity we track or use?

No

Testing instructions:

• Merge this, then have renovate regenerate Update wordpress monorepo #19328, then rebase Update jetpack extension tests to work with update to @wordpress dependencies #20326. See if that fixes the "Linting / ESLint (non-excluded files only)" job.

[github-actions]

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