CI: Cygwin: Cache the downloaded Cygwin packages between runs

Unfortunately, for :reasons: cygwin-install-action doesn't currently do
this itself, so do it explcitly.

(As with the pip cache, use the separate explicit save and restore
actions, so that the save always happens, rather than in a post-run step
which only occurs if the tests pass.)

.github/workflows/cygwin.yml

@@ -36,6 +36,12 @@ jobs:
       MESON_CI_JOBNAME: cygwin-${{ matrix.NAME }}
 
     steps:
+      - uses: actions/cache/restore@v3
+        with:
+          path: C:\cygwin-packages
+          key: cygwin-packages-${{ github.run_number }}
+          restore-keys: cygwin-packages-
+
       - uses: actions/cache/restore@v3
         id: restore-cache
         with:
@@ -76,6 +82,11 @@ jobs:
             vala
             zlib-devel
 
+      - uses: actions/cache/save@v3
+        with:
+          path: C:\cygwin-packages
+          key: cygwin-packages-${{ github.run_number }}
+
       - name: Run pip
         run: |
           export PATH=/usr/bin:/usr/local/bin:$(cygpath ${SYSTEMROOT})/system32