fix(ci): pin astral-sh/setup-uv to immutable v8.1.0

setup-uv stopped publishing moving major tags (``@v8``, ``@v8.0``) at
v8.0.0 for supply-chain hardening, so the previous bump to ``@v8`` failed
with "Unable to resolve action astral-sh/setup-uv@v8, unable to find
version v8". Pin to the immutable ``v8.1.0`` release tag (the
maintainers' recommended pattern) and add a comment pointing at the
release notes for future bumps.

See: https://github.com/astral-sh/setup-uv/releases/tag/v8.0.0
Made-with: Cursor

Author: Yuma-Ichikawa

.github/workflows/ci.yml

@@ -16,8 +16,12 @@ jobs:
     steps:
       - uses: actions/checkout@v5
 
+      # ``setup-uv`` switched to immutable releases at v8.0.0 and stopped
+      # publishing the moving ``@v8`` major tag, so pin to a specific
+      # immutable release. See
+      # https://github.com/astral-sh/setup-uv/releases/tag/v8.0.0
       - name: Install uv
-        uses: astral-sh/setup-uv@v8
+        uses: astral-sh/setup-uv@v8.1.0
         with:
           enable-cache: true