ci: replace oven-sh/setup-bun with curl install for Gitea Actions compatibility

The  action is a GitHub Marketplace action
that doesn't exist on self-hosted Gitea instances. Replace it with
the standard curl-based Bun installation method and add ~/.bun/bin
to GITHUB_PATH.

.gitea/workflows/validate.yml

@@ -14,9 +14,9 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Setup Bun
-        uses: oven-sh/setup-bun@v2
-        with:
-          bun-version: latest
+        run: curl -fsSL https://bun.sh/install | bash
+      - name: Add Bun to PATH
+        run: echo "$HOME/.bun/bin" >> $GITHUB_PATH
 
       - name: Install dependencies
         run: bun install