Add comment explaining why restore-keys is not used

Aligns with the pattern used in other workflows (build_and_deploy.yml,
create_release_branch.yml, trigger_release.yml) that explain restore-keys
is avoided to prevent indefinite cache growth.

Co-authored-by: sebastian.silbermann <sebastian.silbermann@vercel.com>

Author: cursoragent

.github/workflows/build_reusable.yml

@@ -214,6 +214,7 @@ jobs:
         with:
           path: ${{ steps.get-store-path.outputs.STORE_PATH }}
           key: pnpm-store-v2-${{ hashFiles('pnpm-lock.yaml') }}
+          # Do not use restore-keys since it leads to indefinite growth of the cache.
 
       # local action -> needs to run after checkout
       - name: Install Rust