Use actions/cache on Blacksmith runners too

Blacksmith archived useblacksmith/cache in Oct 2025; their runners now
cache transparently and the vendor guidance is to use upstream
actions/cache. This also matches what every ci.yml job already does,
and lets the Linux/macOS cache step split collapse back into one step.

Author: RhysSullivan

.github/workflows/pkg-pr-new.yml

@@ -39,16 +39,6 @@ jobs:
           bun-version: 1.3.11
 
       - name: Cache Bun package cache
-        if: runner.os == 'Linux'
-        uses: useblacksmith/cache@v5
-        with:
-          path: ~/.bun/install/cache
-          key: ${{ runner.os }}-bun-1.3.11-${{ hashFiles('bun.lock') }}
-          restore-keys: |
-            ${{ runner.os }}-bun-1.3.11-
-
-      - name: Cache Bun package cache
-        if: runner.os == 'macOS'
         uses: actions/cache@v4
         with:
           path: ~/.bun/install/cache
@@ -108,7 +98,7 @@ jobs:
           bun-version: 1.3.11
 
       - name: Cache Bun package cache
-        uses: useblacksmith/cache@v5
+        uses: actions/cache@v4
         with:
           path: ~/.bun/install/cache
           key: ${{ runner.os }}-bun-1.3.11-${{ hashFiles('bun.lock') }}

.github/workflows/preview.yml

@@ -36,7 +36,7 @@ jobs:
           bun-version: 1.3.11
 
       - name: Cache Bun package cache
-        uses: useblacksmith/cache@v5
+        uses: actions/cache@v4
         with:
           path: ~/.bun/install/cache
           key: ${{ runner.os }}-bun-1.3.11-${{ hashFiles('bun.lock') }}