CI: cache example .build (experiment - measure warm vs cold)

Author: krodak

.github/workflows/test.yml

@@ -182,6 +182,14 @@ jobs:
       - uses: swiftwasm/setup-swiftwasm@v2
         id: setup-wasm32-unknown-wasip1-threads
         with: { target: wasm32-unknown-wasip1-threads }
+      - uses: actions/cache@v4
+        with:
+          path: |
+            Examples/${{ matrix.example }}/.build
+            ~/.cache/org.swift.swiftpm
+          key: ex-${{ matrix.example }}-${{ hashFiles('.github/workflows/test.yml', format('Examples/{0}/Package.resolved', matrix.example)) }}
+          restore-keys: |
+            ex-${{ matrix.example }}-
       # build.sh resolves the package relative to the working directory, so run it
       # from the example directory (mirroring Utilities/build-examples.sh's `cd`).
       - run: ./build.sh release