CI: Cache example .build to speed up warm example builds

Cache each example's `.build` plus the SwiftPM dependency cache, keyed on the
workflow file and the example's Package.resolved. On a warm cache the matrix
jobs skip recompiling swift-syntax (pulled in by the @js macro plugin and
BridgeJSTool), which dominates a cold example build.

Measured per-example wall-clock on the example matrix:

  no cache:          ~10 min
  cold (miss + save): ~10-12 min
  warm (hit):         ~1-2 min

Cold runs - the first run, or whenever Package.resolved or the workflow file
change - pay a small cache-save overhead. Warm runs, the common case, drop the
example matrix from ~10 min to ~2 min. GitHub's 10 GB cache budget is shared
across the six example `.build` caches, so heavy eviction falls back to 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