ci: detect pnpm from package.json

[AriPerkkio]

• CI is currently showing errors due to conflicting pnpm versions. The lockfile is generated by pnpm@9 but CI is using pnpm@8.

  •  WARN  Ignoring not compatible lockfile at /home/runner/work/eslint-plugin/eslint-plugin/pnpm-lock.yaml
    https://github.com/stackblitz/eslint-plugin/actions/runs/9661791690/job/26650257931

• Marks packageManager into package.json so that pnpm/action-setup can automatically detect it. Similar to ci: detect pnpm version from package.json tutorialkit#101.

I got the 9.4.0 version from:

eslint-plugin/.tool-versions

Lines 1 to 2 in d665151

	pnpm 9.4.0
	nodejs 18.20.3

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation if applicable
• I have added tests for this change

[AriPerkkio]

This has to be run before pnpm/action-setup so that package.json is present on file system and version can be resolved from there.

[d3lm]

So the reason we use pnpm@8 still is that we still need to update pnpm for WebContainer and we are not yet on v9.

[AriPerkkio]

@d3lm do you think we should use pnpm@8 here too? The current lockfile in main is generated with pnpm@9 so we'll need to re-generate that if it's wanted.

[d3lm]

I mean unless we want people to send reproductions with StackBlitz which would be really cool. If yes, then I think we should lock it to the version that we currently support on StackBlitz.com. WDYT?

[AriPerkkio]

Yep that makes sense. I've now lowered the pnpm to v8 and re-generated lockfile.