fix(ci): let pnpm/action-setup read the packageManager field

Specifying version: 10 alongside package.json's packageManager pin makes
pnpm/action-setup fail with conflicting version sources.

Author: timo9378

.github/workflows/flow-diff.yml

@@ -19,8 +19,6 @@ jobs:
           fetch-depth: 0
 
       - uses: pnpm/action-setup@v4
-        with:
-          version: 10
 
       - uses: actions/setup-node@v4
         with: