update-translations.yml: pin astral-sh/setup-uv to v8.1.0

Upstream stopped publishing rolling major/minor tags (v8, v8.0) as
of their v8.0.0 release for supply-chain security reasons, breaking
our workflow which referenced @v8.

.github/workflows/update-translations.yml

@@ -32,7 +32,7 @@ jobs:
 
       # Setting up recommended prerequisites
       # See: i18n/README.md
-      - uses: astral-sh/setup-uv@v8
+      - uses: astral-sh/setup-uv@v8.1.0
       - uses: extractions/setup-just@v4
 
       # TODO: optimize when we start publishing translations and integrate a Weblate instance