ci: add nimble deps caching to Windows build

The Windows build referenced steps.cache-nimbledeps.outputs.cache-hit
but never defined a Cache nimble deps step, so the condition was always
true and nimble deps were never cached on Windows.

Add the cache step with the same versioned key + restore-keys as the
Linux/macOS jobs, and move the .nimble-setup stamp touch into its own
unconditional step so a cache hit doesn't re-trigger `nimble setup`.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/windows-build.yml

@@ -11,6 +11,8 @@ env:
   NPROC: 4
   NIM_VERSION: '2.2.4'
   NIMBLE_VERSION: '0.22.3'
+  # Bump to invalidate the nimble deps cache (GitHub caches are immutable per key).
+  NIMBLE_CACHE_VERSION: 'v2'
 
 jobs:
   build:
@@ -88,6 +90,17 @@ jobs:
       run: |
         sed -i 's/68bb85cbfb1832ce4db43943911b046c3af3caab/a092a045d3a427d127a5334a6e59c76faff54686/g' nimble.lock
 
+    - name: Cache nimble deps
+      id: cache-nimbledeps
+      uses: actions/cache@v3
+      with:
+        path: |
+          nimbledeps/
+          nimble.paths
+        key: ${{ runner.os }}-nimbledeps-${{ env.NIMBLE_CACHE_VERSION }}-nimble${{ env.NIMBLE_VERSION }}-${{ hashFiles('nimble.lock', 'BearSSL.mk', 'Nat.mk') }}
+        restore-keys: |
+          ${{ runner.os }}-nimbledeps-${{ env.NIMBLE_CACHE_VERSION }}-nimble${{ env.NIMBLE_VERSION }}-
+
     - name: Install nimble deps
       if: steps.cache-nimbledeps.outputs.cache-hit != 'true'
       run: |
@@ -95,7 +108,9 @@ jobs:
         nimble setup --localdeps -y
         make rebuild-nat-libs-nimbledeps CC=gcc
         make rebuild-bearssl-nimbledeps CC=gcc
-        touch nimbledeps/.nimble-setup
+
+    - name: Refresh nimbledeps stamp
+      run: touch nimbledeps/.nimble-setup
 
     - name: Creating tmp directory
       run: mkdir -p tmp