Tighten package sources in C# smoketests (#133)

## Description of Changes

Same as clockworklabs/SpacetimeDB#1735 but for
this repo.

## API

 - [ ] This is an API breaking change to the SDK

*If the API is breaking, please state below what will break*


## Requires SpacetimeDB PRs
*List any PRs here that are required for this SDK change to work*

.github/workflows/dotnet.yml

@@ -44,9 +44,18 @@ jobs:
           <packageSources>
             <!-- Local NuGet repositories -->
             <add key="Local SpacetimeDB.BSATN.Runtime" value="../SpacetimeDB/crates/bindings-csharp/BSATN.Runtime/bin/Release" />
-            <!-- Official NuGet.org server -->
-            <add key="NuGet.org" value="https://api.nuget.org/v3/index.json" />
           </packageSources>
+          <packageSourceMapping>
+            <!-- Ensure that SpacetimeDB.BSATN.Runtime is used from the local folder. -->
+            <!-- Otherwise we risk an outdated version being quietly pulled from NuGet for testing. -->
+            <packageSource key="Local SpacetimeDB.BSATN.Runtime">
+              <package pattern="SpacetimeDB.BSATN.Runtime" />
+            </packageSource>
+            <!-- Fallback for other packages (e.g. test deps). -->
+            <packageSource key="nuget.org">
+              <package pattern="*" />
+            </packageSource>
+          </packageSourceMapping>
         </configuration>
         EOF
     - name: Restore dependencies