Install the pinned .NET SDK to a runner-writable dir on self-hosted CI

setup-dotnet defaulted to /usr/share/dotnet, which the runner user cannot
write to (Permission denied). Point DOTNET_INSTALL_DIR at runner.tool_cache
so the 8.0 SDK installs into a writable, cached location.

.github/workflows/csharp-server.yml

@@ -9,6 +9,10 @@ jobs:
   build-and-test:
     name: Repository policy, C# build and test
     runs-on: [self-hosted, Linux, X64]
+    # The self-hosted runner user cannot write to the system /usr/share/dotnet,
+    # so install the pinned SDK into a runner-writable (and cached) path instead.
+    env:
+      DOTNET_INSTALL_DIR: ${{ runner.tool_cache }}/dotnet
     steps:
       - uses: actions/checkout@v4