fix: use workspace-local mise dir on Windows for cache compatibility

@actions/cache uses @actions/glob internally to resolve cache paths,
which fails when the path is on a different drive than GITHUB_WORKSPACE.
On Windows CI runners, GITHUB_WORKSPACE is typically on D: while
LOCALAPPDATA (the previous default) is on C:, causing saveCache to
throw "Path Validation Error" on cold caches.

Default to GITHUB_WORKSPACE/.mise on Windows so the cache path is
always on the same drive as the workspace.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/test.yml

@@ -47,6 +47,7 @@ jobs:
       - name: Setup mise
         uses: ./
         with:
+          cache_key_prefix: mise-debug-v1
           mise_toml: |
             [tools]
             jq = "1.7.1"
@@ -55,6 +56,7 @@ jobs:
             MY_ENV_VAR = "abc"
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          ACTIONS_STEP_DEBUG: true
       - run: mise --version
       - run: mise x jq -- jq --version
       - run: which jq