ci: pin astral-sh/setup-uv to v8.1.0 (full version)

The previous bump used @v8 but astral-sh/setup-uv doesn't publish a
moving major tag — only full SemVer tags (v8.1.0, v8.0.0, etc.). The
test job failed with "Unable to resolve action astral-sh/setup-uv@v8,
unable to find version v8". Pinning to v8.1.0 (the latest released
tag) fixes resolution.

The other three actions (actions/checkout, upload-artifact,
download-artifact) all DO publish moving major tags, so @v6/@v7/@v8
remain valid for those.

Author: chris-colinsky

.github/workflows/ci.yml

@@ -27,7 +27,7 @@ jobs:
           submodules: recursive
 
       - name: Install uv
-        uses: astral-sh/setup-uv@v8
+        uses: astral-sh/setup-uv@v8.1.0
         with:
           enable-cache: true
 

.github/workflows/release.yml

@@ -29,7 +29,7 @@ jobs:
           submodules: recursive
 
       - name: Install uv
-        uses: astral-sh/setup-uv@v8
+        uses: astral-sh/setup-uv@v8.1.0
         with:
           enable-cache: true
 
@@ -86,7 +86,7 @@ jobs:
           submodules: recursive
 
       - name: Install uv
-        uses: astral-sh/setup-uv@v8
+        uses: astral-sh/setup-uv@v8.1.0
         with:
           enable-cache: true