fix(setup/yarn): replace deprecated save-always with cache/restore + cache/save

[james-gre]

Problem

The setup/yarn action uses save-always: true on actions/cache, which has been deprecated (see actions/cache#1315). This produces deprecation warnings in CI logs.

Proposed Fix

Replace the single actions/cache step with the explicit two-step pattern recommended by the actions/cache maintainers:

1. actions/cache/restore — restores cached node_modules at the start
2. actions/cache/save with if: always() — saves the cache even on failure (equivalent behavior to save-always: true)

Diff summary (setup/yarn/action.yml)

-    - name: Restore node_modules
-      uses: actions/cache@9255dc7a253b0ccc959486e2bca901246202afeb #v5.0.1
+    - name: Restore cached node_modules
+      uses: actions/cache/restore@9255dc7a253b0ccc959486e2bca901246202afeb #v5.0.1
+      with:
+        path: |
+          node_modules
+          src/node_modules
+        key: ${{ runner.os }}-yarn-${{ hashFiles('yarn.lock', 'src/yarn.lock') }}
+    - name: Save node_modules cache
+      uses: actions/cache/save@9255dc7a253b0ccc959486e2bca901246202afeb #v5.0.1
+      if: always()
       with:
-        save-always: true
         path: |
           node_modules
           src/node_modules
         key: ${{ runner.os }}-yarn-${{ hashFiles('yarn.lock', 'src/yarn.lock') }}
-        # we faced issues with restoring on partial matches because yarn classic doesn't cleanup properly on install, don't uncomment
-        # restore-keys: ${{ runner.os }}-yarn-

• Same pin: @9255dc7a253b0ccc959486e2bca901246202afeb (v5.0.1)
• Preserves the "always save" behavior without the deprecated parameter
• Removes the stale restore-keys comment (intentionally disabled per prior decision)