Use latest version from manifest-file (#458)

If a manifest-file is supplied the default value of the version input
(latest) will get the latest version available in the manifest. That
might not be the actual latest version available in the official uv
repo.

.github/workflows/test.yml

@@ -540,7 +540,6 @@ jobs:
       - name: Install from custom manifest file
         uses: ./
         with:
-          version: 0.7.12-alpha.1
           manifest-file: "https://raw.githubusercontent.com/astral-sh/setup-uv/${{ github.ref }}/__tests__/download/custom-manifest.json"
       - run: uv sync
         working-directory: __tests__/fixtures/uv-project