ci(pr-checks): pin pnpm — the package carries no packageManager field

The setup step claimed to read the pnpm version from `packageManager` in
package.json, but this plugin package defines no such field, so
pnpm/action-setup aborted with "No pnpm version is specified" eight seconds
into every external PR's run (#102, #103, #107) — no PR check ever reached
the test step. Pin the version explicitly, mirroring release.yml.

.github/workflows/pr-checks.yml

@@ -18,8 +18,11 @@ jobs:
     steps:
       - uses: actions/checkout@v7
 
-      # Reads the pinned pnpm version from `packageManager` in package.json.
+      # No `packageManager` field in this plugin's package.json — pin the
+      # version here, as release.yml does.
       - uses: pnpm/action-setup@v6
+        with:
+          version: latest
 
       - uses: actions/setup-node@v7
         with: