ci: pin astral-sh/setup-uv to v8.1.0

The action does not publish a `v8` major tag (only `v8.0.0` and
`v8.1.0`), so `@v8` fails to resolve in GitHub Actions. The fix
already lives on develop but never reached main before the 3.1.2
hotfix branched off; the v3.1.2 release workflow failed to set up
its jobs. Pin to the exact version on both release.yml and
python-package.yml.

Signed-off-by: Fernando Macedo <fgmacedo@gmail.com>

Author: fgmacedo

.github/workflows/python-package.yml

@@ -30,7 +30,7 @@ jobs:
     - name: Setup Graphviz
       uses: ts-graphviz/setup-graphviz@v2
     - name: Install uv
-      uses: astral-sh/setup-uv@v8
+      uses: astral-sh/setup-uv@v8.1.0
       with:
         enable-cache: true
         cache-suffix: "python${{ matrix.python-version }}"

.github/workflows/release.yml

@@ -24,7 +24,7 @@ jobs:
         uses: ts-graphviz/setup-graphviz@v2
 
       - name: Install uv
-        uses: astral-sh/setup-uv@v8
+        uses: astral-sh/setup-uv@v8.1.0
         with:
           enable-cache: true