Tooling: Fix jetpack cli link and unlink

[tbradsha]

This fixes pnpm jetpack cli link and jetpack cli unlink.

Proposed changes:

In #41821 we upgraded pnpm to v10 and removed the --global flag when linking since it wasn't supposed to need it anymore, but apparently this version didn't work as described. In pnpm v10.5 this was resolved (though that introduced a separate regression resolved in 10.5.2).

For now, we'll just keep the --global for the link. The unlink never seemed to work, and uninstall seems to work just as well, so I changed the script to use that.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

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

Testing instructions:

1. Ensure jetpack isn't in your path/linked/aliased already (e.g. jetpack should give a "not found" error).
2. Run pnpm jetpack cli link.
3. Ensure jetpack runs as expected.
4. Run jetpack cli unlink.
5. Ensure jetpack no longer runs. Note that the error might have residual references to an npm path until you start a new terminal session; fixing that is out of scope and is likely a side effect of pnpm in general.

[github-actions]

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

• ✅ Include a description of your PR changes.
  
• ✅ Add a "[Status]" label (In Progress, Needs Review, ...).
  
• ✅ Add a "[Type]" label (Bug, Enhancement, Janitorial, Task).
  
• ✅ Add testing instructions.
  
• ✅ Specify whether this PR includes any changes to data or privacy.
  
• ✅ Add changelog entries to affected projects
  

This comment will be updated as you work on your PR and make changes. If you think that some of those checks are not needed for your PR, please explain why you think so. Thanks for cooperation 🤖

---

The e2e test report can be found here. Please note that it can take a few minutes after the e2e tests checks are complete for the report to be available.

---

Follow this PR Review Process:

1. Ensure all required checks appearing at the bottom of this PR are passing.
2. Make sure to test your changes on all platforms that it applies to. You're responsible for the quality of the code you ship.
3. You can use GitHub's Reviewers functionality to request a review.
4. When it's reviewed and merged, you will be pinged in Slack to deploy the changes to WordPress.com simple once the build is done.

If you have questions about anything, reach out in #jetpack-developers for guidance!

[zinigor]

Nicely done! Works well, thanks for all the digging you did to get to this solution!