fix: add pnpm version fallback to plugin-ci shared workflow

pnpm/action-setup@v4 requires either a version input or a packageManager
field in package.json. Repos with pnpm-lock.yaml but no packageManager
field were failing with "No pnpm version is specified."

Adding version: latest as a fallback allows the action to install the
latest stable pnpm when packageManager is not set. Repos that do specify
packageManager in package.json continue to use their pinned version.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.github/workflows/plugin-ci.yaml

@@ -32,6 +32,7 @@ jobs:
         uses: pnpm/action-setup@v4
         with:
           run_install: false
+          version: latest
 
       - name: Setup Node
         uses: actions/setup-node@v4