Bump BenchmarkDotNet from 0.12.0 to 0.12.1 in /Ix.NET/Source

Bumps [BenchmarkDotNet](https://github.com/dotnet/BenchmarkDotNet) from 0.12.0 to 0.12.1.
- [Release notes](https://github.com/dotnet/BenchmarkDotNet/releases)
- [Commits](dotnet/BenchmarkDotNet@v0.12.0...v0.12.1)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Ix.NET/Source/Benchmarks.System.Interactive/Benchmarks.System.Interactive.csproj

@@ -20,7 +20,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="BenchmarkDotNet" Version="0.12.0" />
+    <PackageReference Include="BenchmarkDotNet" Version="0.12.1" />
   </ItemGroup>
 
   <ItemGroup Condition="'$(Configuration)|$(Platform)'=='Ix.net 3.1.1|AnyCPU'">