CI: update cygwin cache actions to new version supporting granular save

cache/restore and cache/save now exist, and close the issue linked in
the workflow comment. The new save action runs when invoked, rather than
as a post action.

.github/workflows/cygwin.yml

@@ -35,9 +35,8 @@ jobs:
       MESON_CI_JOBNAME: cygwin-${{ matrix.NAME }}
 
     steps:
-      # cache should be saved on failure, but the action doesn't support that
-      # https://github.com/actions/cache/issues/92
-      - uses: actions/cache@v1
+      - uses: actions/cache/restore@v3
+        id: restore-cache
         with:
           # should use 'pip3 cache dir' to discover this path
           path: C:\cygwin\home\runneradmin\.cache\pip
@@ -86,6 +85,12 @@ jobs:
           python3 -m pip --disable-pip-version-check install gcovr jsonschema pefile pytest pytest-subtests pytest-xdist coverage codecov
         shell: C:\cygwin\bin\bash.exe --noprofile --norc -o igncr -eo pipefail '{0}'
 
+      - uses: actions/cache/save@v3
+        with:
+          # should use 'pip3 cache dir' to discover this path
+          path: C:\cygwin\home\runneradmin\.cache\pip
+          key: cygwin-pip-${{ github.run_number }}
+
       - name: Run tests
         run: |
           export PATH=/usr/bin:/usr/local/bin:$(cygpath ${SYSTEMROOT})/system32