ci: Update pnpm message in check-lock-files.sh

[anomiex]

Proposed changes:

It doesn't matter which directory you run pnpm install in. That's left over from when we still used yarn without workspaces.

OTOH, there are some cases where pnpm install doesn't make a change but pnpm install --resolution-only does. Suggest the latter in case the former fails to make the needed change.

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

#38146 (comment)

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

No

Testing instructions:

• CI happy?
• You might try making a change based on this one where you update a package.json dep version without running pnpm install.

[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:

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