Bump BenchmarkDotNet from 0.12.0 to 0.12.1 in /Rx.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](https://github.com/dotnet/BenchmarkDotNet/compare/v0.12.0...v0.12.1)
Signed-off-by: dependabot-preview[bot] <[email protected]>