ci: install .NET via retrying dotnet-install.sh instead of setup-dotnet

The single self-hosted act_runner intermittently fails actions/setup-dotnet:
a partial extraction sticks in the shared tool-cache (tar: Cannot open: File
exists) or the SDK tarball download corrupts. Install into a clean private
$HOME/.dotnet via dotnet-install.sh with a rm -rf + retry-once, matching the
npm ci and NuGet publish retries already in this pipeline.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/ci-deploy.yml

@@ -13,10 +13,22 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
-      - name: Setup .NET
-        uses: actions/setup-dotnet@v4
-        with:
-          dotnet-version: '9.0.x'
+      - name: Setup .NET (resilient)
+        shell: bash
+        # actions/setup-dotnet on this single self-hosted runner intermittently
+        # leaves a partial extraction in the shared tool-cache ("tar: Cannot open:
+        # File exists") or corrupts the SDK download. Install into a clean private
+        # dir via dotnet-install.sh and retry once on failure, mirroring the
+        # npm ci / NuGet retries elsewhere in this workflow.
+        run: |
+          install() {
+            curl -fsSL https://dot.net/v1/dotnet-install.sh -o /tmp/dotnet-install.sh
+            rm -rf "$HOME/.dotnet"
+            bash /tmp/dotnet-install.sh --channel 9.0 --install-dir "$HOME/.dotnet"
+          }
+          install || ( echo "dotnet install failed ($?) — retrying once..." && install )
+          echo "$HOME/.dotnet" >> "$GITHUB_PATH"
+          "$HOME/.dotnet/dotnet" --info
 
       - name: Setup Node
         uses: actions/setup-node@v4